Warning: I am NON-Technical !
Similar problem here (SuSE Linux Enterprise and openSUSE) with GNOME Desktop from time to time.
Solution for self was to select: "Switch User"
Log in as another user, then login as root, where can boot to closed the locked open session.
Alternately, after look up erring users password, log out, log in as them, then log them out.
Recall being able to leave them a message for when they next logged in.
As last resort, read my manual :-O
27.4 Login Problems