Can you give a link to the page you're actually seeing this error on? I'm not sure if it's the URL in the screencast, so just want to make sure we're debugging the right issue.
Generally, there's a few causes for this..
1) Your Site URL and Base URL are not correctly setup in your Facebook application.
2) Most/all pages on your site have redirections. This can confuse Facebook, and generally can happen if your entire site requires a user to be logged in. Mainly, the home page shouldn't redirect to a login page, when possible.
3) There's an incorrect Open Graph URL tag on the page that's conflicting with the URL set in the Comments module
Again, with the URL this problem occurs on, I can check most of the above, or give you advice on how things should be considered.
Thanks,
Alex
URL Unreachable Error on JFBComments
