On this support thread (http://forum.coppermine-gallery.net/index.php?topic=32342.20), and in particular this post (http://forum.coppermine-gallery.net/index.php?topic=32342.msg151660#msg151660), it was reported that a user who has a non-administrator primary membership, but administrator as a secondary group membership, is not given administrator privileges on the gallery.
I or someone else should investigate this and either remove Administrator as a possible secondary group membership checkbox (and on the group manager) or implement this correctly to make sure an administrator is an administrator no matter if primary or secondary.
Secondary group functionality has always been confusing and I do wonder if it does work the way it is percieved to...
I would go with Administrator cannot be a secondary group
Removed admin from secondary group list in usermgr.php.
Change committed to devel and stable. Will be in the next version: 1.4.9.