Actually, it was a bit more subtle than caching even.
I tried to reproduce my steps, and accidentally found out what i did that caused this. When logged on as admin, i first set my personal theme different from the forum setting. Then i went to configuration, and disabled user selectable themes. However i did not change the theme in the configuration. So after getting back to the forum, it was still in my personal theme..... even after log off - log on.
So then i tried it again, but now, i also changed the theme setting in forum configuration. Now, my personal theme changed to the forum theme. So, as far as i can deduct, just unchecking the option for user selectable skins does not reset skins for users. However, if you select another skin after that, the user skins will reset....
Can you confirm that this is the way it works, and that it should work this way?
cheers,
erik