First, make sure the New User Redirection page setup in the JFBConnect config area is a valid page. Mainly, that it's pointing at a real menu item that is has a status of Public or Registered.
If that's setup correctly, can you try disabling any SEF components you have installed, and see if you're still getting the issue. Also, let us know what SEF extensions you have installed, as that may help us with determining the cause (and giving an answer).
If that still doesn't solve it, let us know, and we can gladly look further into it for you. Usually, this is an SEF URL related issue though, and we've always been able to solve those once we know a bit more about your configuration.