Your Web server does not support, or refuses to support, the HTTP protocol version specified by the client (e.g. your Web browser in the HTTP request data stream sent to the server.
The HTTP protocol has various versions identified as major.minor e.g. version 0.9, 1.0 or 1.1. Your server is indicating that it is unable or unwilling to complete the request using the major version provided by the client - other than with this error message.
Assuming that your request identifies a valid major.minor version number (the request is not fundamentally corrupt), then this error should mostly only occur if you are trying to use version 1.0 or 1.1, but your Web server only supports the older 0.9 version.
When you use Internet Explorer on your Windows XP-based computer to try to view a Web page, you may receive an error message that is similar to the following:
Error 501/505: Not implemented or not supported.
505 is a error that is signaled by browser when HTTP version is not supported by server i.e. when server not able to identify the HTTP protocol version used in request.
All errors contained in 5xx list are server errors >>>
500 (server unable to identify error) “ internal server error” is signaled by browser when Server is not able to identify which particular message is is suitable to to be singled i.e. generic error messages.
501(server unable to execute request) “not implemented” is signaled by browser when server is enable toidentify goal of request or not have enough enough power to execute request.
502 ( server detect invalid response) “bad gateway” is signalled by Browser when an invalid response received by server when server act as gateway or proxy.
503 ( server overloaded ) “service unavailable” is signaled by browser when server is overloaded or down due to excessive or too much number of users’ involvement.
504 ( server reached response time limit) “gateway timeout” when server is unable to identify the request in his fixed time limit when server act as gateway or proxy.
511 ( server demands user to authenticate) “ network authentication required ” when server needs user authentication to access request.more...