Just circling back to it again and running through the registration: it keeps telling me that "John Smith" is an invalid user name. There are no hints anywhere on the login form that the name cannot contain spaces, so the user has to figure this out somehow.
Can you either allow full user names in any format they wish, or add a note saying that it cannot contain spaces (and whatever length restrictions are), right on the registration form?
Or at the very least change the error message.