Did you configured anything differently recently?
I'm not sure what "CommunityIndex.php" is, but Community is usually JomSocial related. Not saying that's the cause problem, but do you have that installed? That can help narrow things down.
The other idea is to possibly change the ordering of the User, Authentication, or System plugins. Not sure what the best combination would be, but ordering each higher or lower than others may change the behavior. No authentication, system, or user plugins should be redirecting the user doing the login process, as that would interrupt JFBConnect's full login process. Finally, the last thing to try is turning off any SEF extensions you may have installed to see if that improves things. If so, we'll help you figure out how to re-enable them.
Thanks,
Alex