Microsoft JScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server...

//catch timeout error
        Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
        function EndRequestHandler(sender, args) {
            if (args.get_error() != undefined) {
                var errorMessage;
                var error = args.get_error;
                if (args.get_response().get_statusCode() == '12031') {    //ERROR_INTERNET_CONNECTION_RESET
                    errorMessage = "Session Timeout.";
                }
                else if (args.get_response().get_statusCode() == '200') {
                    errorMessage = args.get_error().message;
                }
                else {
                    errorMessage = 'An unspecified error occurred. ';
                }
                args.set_errorHandled(true);
                alert("An error occurred: " + errorMessage + " Please refresh the page to restart your session.");
               // window.location.reload();  //refresh to re-login
            }
        }