From last Update with MacOS (Intelbased) and Problems i know a workaround.
And yes, this works with BigSur, too.
1. uninstall Forticlient.
3. now, you can find fragmented files of Forticlient in /Library/StageExtensions Folder. You have to delete them. But this did not work without administration access in terminal window. So, open Terminal Window and paste:
sudo kextcache --clear-staging
After this, enter your Computer Password (administration rights need)
5. now install old Forticlient Version: FortiClient_22.214.171.1243_macosx.dmg
7. now you can install newer Forticlient Version: FortiClient_126.96.36.199_macosx.dmg
After that you can config all SSL VPN Tunnel and IPsec Tunnel,
they worked as well as before Update to MacOS big sur.
Tested on 4 different MACs. Works fine.
Best wishes out of Germany