Hier die häufigsten HTTP-Fehlercodes und ihre Bedeutung:

302: Vorrübergehend verschoben

400: Fehlerhafte Anfrage

401: Nicht authorisiert (Unauthorized)
Sie rufen wahrscheinlich eine Seite auf, die geschützt ist und zu der Sie keinen Zugang haben oder Sie haben sich bei der Eingabe des Passwortes vertippt. Behebung: Wenn Sie sicher sind, dass Ihnen der Zugang möglich ist, versuchen Sie es erneut. Achten Sie bei der Eingabe des Passwortes auf Groß- und Kleinschreibung.

403: Verboten (Forbidden)
Diese Seiten sind (anders wie bei einem 401) Ihnen nicht zugänglich, da Sie beispielsweise von einem unbekannten System aus zugreifen oder die Seiten einfach generell nicht verfügbar sind. Behebung: Keine

404: Nicht gefunden (Not Found)
Der Server konnte die Datei nicht finden, das Sie angefordert haben.Es kann sein, dass Sie sich bei der Eingabe der Adresse verschrieben haben. Möglich wäre auch, dass das Dokument nicht mehr existiert, der Name geändert wurde oder es an einer anderen Stelle steht. Behebung: Gehen Sie eine Ebene zurück, d.h. löschen Sie den letzten Teil der Adresse bis zum nächsten Querstrich ("/") und fordern Sie diese Seite an. Vielleicht finden Sie hier Hinweise auf den Verbleib des Dokuments. Wenn das nicht funktioniert, rufen Sie NUR den Domainnamen auf. Beispiel: http://www.musterfrau.de/outlook/blabla.html wird nicht gefunden. Probieren Sie (1. Schritt): http://www.musterfrau.de/outlook und schlussendlich: http://www.musterfrau.de

500: Interner ServerFehler (Bad Script Request)
Das Skript, das auf dem Server läuft, konnte nicht ausgeführt werden. Diese Meldung tritt zum Beispiel auf, wenn ein Perl-Skript entweder syntaktisch nicht korrekt ist oder die Ausführrechte nicht vergeben wurden.

503: Vorübergehend nicht erreichbar

Weitere Fehlercodes (leider nur in Englisch)

This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line. Since HTTP/1.0 did not define any 1xx status codes, servers should not send a 1xx response to an HTTP/1.0 client except under experimental conditions.

100 Continue
The client may continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client should continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server must send a final response after the request has been completed.

101 Switching Protocols
The server understands and is willing to comply with the client's request, via the Upgrade message header field (Section 10.41), for a change in the application protocol being used on this connection. The server will switch protocols to those defined by the response's Upgrade header field immediately after the empty line which terminates the 101 response.
The protocol should only be switched when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over older versions, and switching to a real-time, synchronous protocol may be advantageous when delivering resources that use such features.

9.2 Successful 2xx
This class of status code indicates that the client's request was successfully received, understood, and accepted.

200 OK
The request has succeeded. The information returned with the response is dependent on the method used in the request, as follows:

GET
an entity corresponding to the requested resource is sent in the response;
HEAD
the response must only contain the header information and no Entity-Body;
POST
an entity describing or containing the result of the action;
TRACE
an entity containing the request message as received by the end server;
otherwise,
an entity describing the result of the action;
If the entity corresponds to a resource, the response may include a Location header field giving the actual location of that specific resource for later reference.

201 Created
The request has been fulfilled and resulted in a new resource being created. The newly created resource can be referenced by the URI(s) returned in the URI-header field and/or the entity of the response, with the most specific URL for the resource given by a Location header field. The origin server should create the resource before using this Status-Code. If the action cannot be carried out immediately, the server must include in the response body a description of when the resource will be available; otherwise, the server should respond with 202 (accepted).

202 Accepted
The request has been accepted for processing, but the processing has not been completed. The request may or may not eventually be acted upon, as it may be disallowed when processing actually takes place. There is no facility for re-sending a status code from an asynchronous operation such as this.
The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The entity returned with this response should include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled.

203 Non-Authoritative Information
The returned metainformation in the Entity-Header is not the definitive set as available from the origin server, but is gathered from a local or a third-party copy. The set presented may be a subset or superset of the original version. For example, including local annotation information about the resource may result in a superset of the metainformation known by the origin server. Use of this response code is not required and is only appropriate when the response would otherwise be 200 (ok).

204 No Content
The server has fulfilled the request but there is no new information to send back. If the client is a user agent, it should not change its document view from that which caused the request to be generated. This response is primarily intended to allow input for actions to take place without causing a change to the user agent's active document view. The response may include new metainformation in the form of entity headers, which should apply to the document currently in the user agent's active view.
The 204 response must not include an entity body, and thus is always ternminated by the first empty line after the header fields.

205 Reset Content
The server has fulfilled the request and the user agent should reset the document view which caused the request to be generated. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action. The response must include a Content-Length with a value of zero (0) and no entity body.

206 Partial Content
The server has fulfilled the partial GET request for the resource. The request must have included a Range header field (Section 10.33) indicating the desired range. The response must include a Content-Range header field (Section 10.14) indicating the range included with this response. All entity header fields in the response must describe the actual entity transmitted rather than what would have been transmitted in a full response. In particular, the Content-Length header field in the response must match the actual number of OCTETs transmitted in the entity body. It is assumed that the client already has the complete entity's header field data.

9.3 Redirection 3xx
This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. The action required may be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A user agent should never automatically redirect a request more than 5 times, since such redirections usually indicate an infinite loop.

300 Multiple Choices
The requested resource is available at one or more locations and a preferred location could not be determined via preemptive content negotiation (Section 12). Unless it was a HEAD request, the response should include an entity containing a list of resource characteristics and locations from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice may be performed automatically. If the server has a preferred choice, it should include the URL in a Location field; user agents not capable of complex selection may use this field value for automatic redirection. This response is cachable unless indicated otherwise.

301 Moved Permanently
The requested resource has been assigned a new permanent URI and any future references to this resource should be done using one of the returned URIs. Clients with link editing capabilities should automatically relink references to the Request-URI to one or more of the new references returned by the server, where possible. This response is cachable unless indicated otherwise.
If the new URI is a single location, its URL must be given by the Location field in the response. If more than one URI exists for the resource, the primary URL should be given in the Location field and the other URIs given in one or more URI-header fields. Unless it was a HEAD request, the Entity-Body of the response should contain a short hypertext note with a hyperlink to the new URI(s).

If the 301 status code is received in response to a request other than GET or HEAD, the user agent must not automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.

302 Moved Temporarily
The requested resource resides temporarily under a different URI. Since the redirection may be altered on occasion, the client should continue to use the Request-URI for future requests. This response is only cachable if indicated by a Cache-Control or Expires header field.
If the new URI is a single location, its URL must be given by the Location field in the response. If more than one URI exists for the resource, the primary URL should be given in the Location field and the other URIs given in one or more URI-header fields. Unless it was a HEAD request, the Entity-Body of the response should contain a short hypertext note with a hyperlink to the new URI(s).

If the 302 status code is received in response to a request other than GET or HEAD, the user agent must not automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.

303 See Other
The response to the request can be found under a different URI and should be retrieved using a GET method on that resource. This method exists primarily to allow the output of a POST-activated script to redirect the user agent to a selected resource. The new resource is not a replacement reference for the original Request-URI. The 303 response is not cachable, but the response to the second request may be cachable.
If the new URI is a single location, its URL must be given by the Location field in the response. If more than one URI exists for the resource, the primary URL should be given in the Location field and the other URIs given in one or more URI-header fields. Unless it was a HEAD request, the Entity-Body of the response should contain a short hypertext note with a hyperlink to the new URI(s).

304 Not Modified
If the client has performed a conditional GET request and access is allowed, but the document has not been modified since the date and time specified in the If-Modified-Since field, the server must respond with this status code and not send an Entity-Body to the client. Header fields contained in the response should only include information which is relevant to cache managers or which may have changed independently of the entity's Last-Modified date. Examples of relevant header fields include: Date, Server, Content-Length, Content-MD5, Content-Version, Cache-Control and Expires.
A cache should update its cached entity to reflect any new field values given in the 304 response. If the new field values indicate that the cached entity differs from the current resource (as would be indicated by a change in Content-Length, Content-MD5, or Content-Version), then the cache must disregard the 304 response and repeat the request without an If-Modified-Since field.

The 304 response must not include an entity body, and thus is always ternminated by the first empty line after the header fields.

305 Use Proxy
The requested resource must be accessed through the proxy given by the Location field in the response. In other words, this is a proxy redirect.

9.4 Client Error 4xx
The 4xx class of status code is intended for cases in which the client seems to have erred. If the client has not completed the request when a 4xx code is received, it should immediately cease sending data to the server. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method.

Note: If the client is sending data, server implementations on TCP should be careful to ensure that the client acknowledges receipt of the packet(s) containing the response prior to closing the input connection. If the client continues sending data to the server after the close, the server's controller will send a reset packet to the client, which may erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application.
400 Bad Request
The request could not be understood by the server due to malformed syntax. The client should not repeat the request without modifications.

401 Unauthorized
The request requires user authentication. The response must include a WWW-Authenticate header field (Section 10.44) containing a challenge applicable to the requested resource. The client may repeat the request with a suitable Authorization header field (Section 10.6). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user should be presented the entity that was given in the response, since that entity may include relevant diagnostic information. HTTP access authentication is explained in Section 11.

402 Payment Required
This code is reserved for future use.

403 Forbidden
The server understood the request, but is refusing to fulfill it. Authorization will not help and the request should not be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it should describe the reason for the refusal in the entity body. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.

404 Not Found
The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent. If the server does not wish to make this information available to the client, the status code 403 (forbidden) can be used instead. The 410 (gone) status code should be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.

405 Method Not Allowed
The method specified in the Request-Line is not allowed for the resource identified by the Request-URI. The response must include an Allow header containing a list of valid methods for the requested resource.

406 None Acceptable
The server has found a resource matching the Request-URI, but not one that satisfies the conditions identified by the Accept and Accept-Encoding request headers. Unless it was a HEAD request, the response should include an entity containing a list of resource characteristics and locations from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice may be performed automatically.

407 Proxy Authentication Required
This code is similar to 401 (unauthorized), but indicates that the client must first authenticate itself with the proxy. The proxy must return a Proxy-Authenticate header field (Section 10.30) containing a challenge applicable to the proxy for the requested resource. The client may repeat the request with a suitable Proxy-Authorization header field (Section 10.31). HTTP access authentication is explained in Section 11.

408 Request Timeout
The client did not produce a request within the time that the server was prepared to wait. The client may repeat the request without modifications at any later time.

409 Conflict
The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user may be able to resolve the conflict and resubmit the request. The response body should include enough information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user-agent to fix the problem; however, that may not be possible and is not required.
Conflicts are most likely to occur in response to a PUT or PATCH request. If versioning is being used and the entity being PUT or PATCHed includes changes to a resource which conflict with those made by an earlier (third-party) request, the server may use the 409 response to indicate that it can't complete the request. In this case, the response entity should contain a list of the differences between the two versions in a format defined by the response Content-Type.

410 Gone
The requested resource is no longer available at the server and no forwarding address is known. This condition should be considered permanent. Clients with link editing capabilities should delete references to the Request-URI after user approval. If the server does not know, or has no facility to determine, whether or not the condition is permanent, the status code 404 (not found) should be used instead. This response is cachable unless indicated otherwise.
The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as "gone" or to keep the mark for any length of time -- that is left to the discretion of the server owner.

411 Length Required
The server refuses to accept the request without a defined Content-Length. The client may repeat the request if it adds a valid Content-Length header field containing the length of the entity body in the request message.

412 Unless True
The condition given in the Unless request-header field (Section 10.40) evaluated to true when it was tested on the server and the request did not include a Range header field (which would indicate a partial GET) or an If-Modified-Since header field (which would indicate a conditional GET). This response code allows the client to place arbitrary preconditions on the current resource metainformation (header field data) and thus prevent the requested method from being applied to a resource other than the one intended.

9.5 Server Error 5xx
Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has erred or is incapable of performing the request. If the client has not completed the request when a 5xx code is received, it should immediately cease sending data to the server. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These response codes are applicable to any request method and there are no required header fields.

500 Internal Server Error
The server encountered an unexpected condition which prevented it from fulfilling the request.

501 Not Implemented
The server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.

502 Bad Gateway
The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.

503 Service Unavailable
The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay may be indicated in a Retry-After header. If no Retry-After is given, the client should handle the response as it would for a 500 response.

Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.
504 Gateway Timeout
The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server it accessed in attempting to complete the request.

Quelle   http://www.w3.org