HTTP 501 status code

501 Not Implemented

The server does not support a function required by the current request. When the server does not recognize the requested method and cannot support its request for any resource.

General methods for solving 501 errors

This error should be very rare in any browser. More likely if the client is not a browser - especially if the web server is old. In both cases, if the client specifies a valid request type, the web server either responds incorrectly or needs to be upgraded.