Your test account made it through. Not sure if that is any help.
Now that you say that, I wonder if the BP template overload plugin has anything to do with the issue? Strangely enough, the registration error started occurring with the Buddypress update.
Give me a moment and I can give you access.
This is a link to the staging site.
I will also need to be able to approve site members before they gain access to the site.