We use windows active directory for login credentials to our PDM Pro.  I am able to add the username in question to the archive server just fine.  When I login to the PDM Admin console -->  Add Users, the user's login name appears in the list, so clearly it is available to add, but when I select the user and click 'Next' I get a dialog stating "The file vault has already got a user with the login name 'Login_Name'."

And no, the user is not listed under 'Users' in the Admin console.