To resolve this:
1. Verify if you have an entry for the affected user in the Group_ table but not in the User_ table:
select * from Group_
select * from User_
If so procede:
2. stop the UMP wasp
delete from Group_ where userid = '<affected_user_id>'
3. start wasp
4. log in as admin > control panel > server administration > reindex all search indexes
5. log out and log in as <user>.