It looks like your "Invite Action Url" is not set. This should point to your homepage, or to another page within your site that you'd want the 'invitees' to be directed to (i.e. "Welcome Facebook Users! Here's why our site is so great...."). I'm not sure if this would cause the problem you're having, but it may, since FB wouldn't know where to send the user now that they've accepted your app... may just think it's a bogus invite.
Also, there's a typo in our code, which may be causing it, but since it's not affecting anyone else, I don't think that's the case. We will be posting a new Invite module shortly with the fix, and if the above doesn't work for you, we'll post what you need to do here. Just let us know.