Probably the referrer checking and your security settings.
You can probably update the line:
if ((($_SERVER["REQUEST_METHOD"] == "POST") && (isset($_SERVER["HTTP_REFERER"]) && strpos(urldecode($_SERVER["HTTP_REFERER"]), urldecode($_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"])) > 0) && isset($_POST))) {
with the line:
if (isset($_POST["Register_x"]))
That should stop the referrer validation.