I just registered on your site on the page you linked to above without issue. Are you sure the email address of the Facebook user you're trying to connect with isn't already in your Joomla database.. and that user isn't blocked? Also, check the JFBConnect - UserMap area to make sure there isn't a mapping for that Facebook user to any user on your Joomla site, and if so, that that user is authorized to access the site.
Hope that helps, but if not, let us know.
Thanks,
Alex