The best suggestion I can give when this happens is the following:
- Re-create your Facebook application leaving all values at their default.
- Set your Canvas Callback Url and Connect Url to your home page
- Get the API/Secret keys and put them into JFBConnect and verify it works.
We've given this suggestion in numerous other posts, and people have always said it works. It seems there's some setting that causes this in the Facebook application. We haven't been able to find it, but if you get it to work using the above, and then want to customize your application further, let us know if you ever find the magic switch causing this. We'd love to test further and provide a workaround (if possible) to avoid this in the future.