Last tuesday Oct. 11, users began receiving error -5 right after providing Forticlient with user/pass. The error occurs before the 2FA challenge appears. The status % of Forticlient at this time is 40%.
Windows 11 had just pushed updates
Security Update for Microsoft Windows (KB5017328)
Security Update for Microsoft Windows (KB5018418)
Servicing Stack 10.0.22000.1035
which appeared to be the central problems. Many users uninstalled KB5017328 and KB5018418 and could then pause Windows updates and Forticlient began logging in successfully.
However some users receiving error -5 do not have any of these updates installed (let alone the fact that we can't uninstall Servicing Stack updates).
In these cases, these users are stuck and have no way to connect.