Error 503 Service Unavailable indicates that the webserver is temporarily unable to process the request. This can be a web server that you are trying to access directly, or another server, which in turn is trying to access a web server. It is called error 503 because it is the HTTP status code used by the webserver to detect such an error. The error can occur for several reasons, but the two most common reasons are: the server is overloaded with requests or maintenance is being performed.