We don't know your site, so I can't give a specific way to test. What I would recommend is disabling extensions one by one until the login process starts working. Then, re-enable the extensions (plugins, modules, etc) until it breaks again. Then, you should know what extension is causing the problem and then narrow down features in that extension (by changing settings until it works, etc) and following the same steps as above.
Again, we'd recommend disabling the JFBCSystem plugin first and the SCLogin module first, since that will completely removed JFBConnect and our login module from the possible-cause list. If you think it's our products, again, we'll gladly help, but we don't know every extension out there. Once you narrow it down with the above, you'd need to contact the developer of that extension.
Another possibility is searching for, and posting, your problem in the general Joomla support forums. It's possible someone else has run into this problem, but it's not something we've heard of:
forum.joomla.org
I hope that helps,
Alex
Logout redirect to 404 page
