This is probably due to the trigger we are using. We do referrer checking as part of the security routine to make sure the page was submitted to itself to prevent form spoofing.
Sometimes (I'm guessing in your case) firewalls or security settings block the referrer from being read in.
Solve the problem by replacing:
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:
if ((($_SERVER["REQUEST_METHOD"] == "POST") && isset($_POST) && isset($_POST["Register"]))) {