Not totally sure. Since you bounced db, can't be anything with open sessions etc. Maybe an export of the user and import back, then dropping.. just guessing here, since it looks like some corruption at OS level or something.
I dropped the default_tablespace for this user, it still does not work. By the way, I have the problem with dropping queues owned by this user. I will try to exp and imp the user and try again see if it helps. Thanks all of you.