The HyperText Transfer Protocol (HTTP) 504 Gateway Timeout
server error response code indicates that the server, while acting as a gateway or proxy, did not get a response in time from the upstream server that it needed in order to complete the request.
Note: A Gateway might refer to different things in networking and a 504 error is usually not something you can fix, but requires a fix by the web server or the proxies you are trying to get access through.