The function group_user_del() removes user(s) from a group, it doesn't check if the user(s) are members of some other group though, in this case removing them leads to have user(s) group orphaned but still active, with no permissions at all.

To reproduce:

 * take an user from the registered user group which is not member of any other group.

 * remove it from the registered user group.

 * login as the removed user and test the board.