If you have FSSO in place, perhaps it isn't able to properly communicate with your DCs and that is why only Administrator is working? That kind of makes sense if you are launching the browser with Domain Admin rights (actually this is a nightmare, never do this!), but if you are speaking about local administrator rights then perhaps a dead end. You can copy your policy that uses authentication and remove that feature, place it above your current policy and disable the one that isn't working until you are able to better review your Fortigate and or FSSO logs. I'd open a case with the TAC, they should be able to help.
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