Sorry - in the original post I meant UPN logins not SPN logins at the ADFS prompt. I've edited it.
The setup and testing is all being done with AD accounts. The only FileVista account is the original admin which I'm not using.
The internal users are logging on with your login page using the Domain\Username format with their AD credentials. They have a corresponding FileVista account with FileVista group permissions to the root folders, and AD permissions on the associated server shares.
The external users are logging in with WAP/ADFS using the UPN format and their AD credentials. They are automatically logged in to FileVista and don't see your login page. I didn't do anything to set this up, it just happens and was a pleasant surprise when I first tested because I didn't realise FileVista was claims aware.
Setting ConnectAs=Authenticated User (Windows) on the root folders causes internal users to get the "161 Specified Path Invalid" error, as well as the external users. If I'm logged on internally as a Domain Admin and set ConnectAs=Authenticated User (Windows) I notice that CurrentUser=(not found). Shouldn't the domain users name appear here?