I have never personally experience this issue directly, but it's not uncommon for such issues to be the result of browser compatibility issues (try switching browsers and/or clearing browser cache, resetting browser to defaults).
Just deployed some new 200Ds and migrated several 200B to 5.0.8 (wireless controller ver) and 5.0.9 -- Encountered 2 GUI-related issues so far (on the same 200D running 5.0.8 wireless controller build); I'm running the latest Firefox browser.
First "bug" is trying to add a new entry to a DHCP-reserved IP list that already has about 30 entries, clicking apply gives an error message then it wipes out the entire reserved IP list -- not a happy camper (had to copy/paste the IP list from an old backup into the CLI to restore the entire list). Fortigate was no where near the max 200 reserved IPs as pointed out by the Maximum values table.
Second "bug" is related to the Block Intra-SSID traffic option on the SSID setting -- I just set up a new SSID, on a separate subnet, and enabled this option. But later was told by the site admin that they can't print to their wireless printer, so I unchecked this option (from the GUI). Site admin reports printer is still unreachable, nor even pingable. I was able to ping all devices on that subnet from the Fortigate, but not from a test machine (via remote connection). So I finally checked the setting via the CLI and discovered the option was still enabled even though the GUI shows it unchecked. >:(
I like stories. :)