I am having the same problem. I get a page error after user fills in details. I can register them from the Admin side, but they cant register at all.
The website encountered an error while retrieving http://www.thissabbath.com/register/. It may be down for maintenance or configured incorrectly.
Here are some suggestions:
Reload this web page later.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfil the request.
BP – 1.6.4
WP – 3.5.1