A 501 is HTTP status code for not implemented. This status code is received when the server does not support the facility required.
Web server overload
If the server that is handling the web page you're attempting to load is experiencing high traffic or is otherwise overloaded, it may be unable to complete your request and generate this error. If you're experiencing this error on a known good page, try loading the page again in a few minutes. If that is unsuccessful, try loading the page again later that day, or the next day.
Web server errors
This error can also be encountered when the web server does not properly respond to the web browser's request, or program attempting to make a request with the web server.
Web server out of date
Finally, in some rare situations, the web server may need a software upgrade in order to communicate with your web browser or program properly.