Can you provide the URL you are seeing this error on (You can message it to us if you'd prefer not to make it public)
Also:
- Is this browser specific? What browser are you seeing it in?
- In the JFBConnect admin area, does it show any errors related to your Facebook application?
That's an unfamiliar error to us. Is the redirect page you're trying to use part of the same domain as the one you are trying to log in from?