Web GUI inaccessible

Yesterday, suddenly our Fortigate 200D became inaccessible. I can still PING the WAN IP, I can still see the username and password prompt but as soon as I logged in, it just shows me HTTP 400 or sometime shows me an error button with the partial GUI image of the device, as soon as I click on ok under error, it just logs me out again. 

The solution was no choice but to reboot the appliance since it has been affecting our services. Our services became slow and after the reboot, it did solve the issue. 

What could be the cause of this HTTP 400? is this an firmware issue?

Thanks

Jeff