Have reviewed your install and there's some very major URL rewriting issues going on. For example your "Join Now" menu item trys to go to CBs profile page even though it's URL is to CBs registration. Your "Member Login" just goes to a broken page even though it's pointing to CBs login page. I was able to get through a few times after disabling all of the SEF, but it looks like JoomSEF is causing page URL rewrite issues and am surprised you can even reach registration or even manage to have it submit.
Please disable all SEF, all caching, purge all caching, then re-test and see if your issue persists.