Does it happen when you log out of the front-end and admin area of the site?
The best recommendations I can give are to try rearranging the JFBCSystem and JFBCUser plugins to different orders. Try making different combinations of making either first or last, and see if that helps.
We'll try to do more investigation on our end, but we do a lot of testing with almost that exact configuration and haven't seen that problem and it's never been reported before.
Thanks,
Alex