Users hidden while they shouldn't be

On my Board users in the userlist appear now as hidden sometimes while they don't have the rights.
This happens to regsitrated users and admins. This i noticed since the last couple of days.

Checked things:

Settings users.
changing settings of hidden/not hidden with no effect.
Changed rights for groups to never.
Cleared buffer.
values database.

All with no effect. And i have no idea what this could be.