Hi guys, got this very same problem. But, found out the/a reason - seems to be a bug or smth in the Dashboard Widget for Gateways.
Yesterday I set up three different gateway groups (Multi wan), and suddenly I got this CSRF check failed thing. Well, after a while I managed to go directly to another page within the firewall gui, that is not index.php, and that worked. So, from there I went straight to System - Routing - Groups and deleted the gateway groups I made, and voila! Back to normal.
Don't know if your problem got the same reason behind… but maybe?