A 502 error is generated when a gateway or proxy server receives an invalid response from an upstream server in its attempt to fulfill a URL access request from the user. Typically, a 502 error indicates a protocol error rather than an upstream server being down.
A protocol error can occur between a gateway server and an upstream server during the data exchange process. When a 502 error occurs, one or both machines may have been programmed incorrectly and did not follow general Internet protocols. To see if the problem is temporary, a browser's data cache can be cleared before attempting to access the website again.