This is a problem of the user management tool of GNOME. It does not correctly add the users to the desired groups. Please check your /etc/group file to see whether the desired users are really in the lpadmin group. If not, add them by editing the file manually or using another user management tool.
CUPS and system-config-printer (gnome-cups-manager is obsolete) are working correctly. All users in the lpadmin group can do printer administration tasks without further authentication. In the CUPS web interface user name and password have to be from any user in the lpadmin group.
This is a problem of the user management tool of GNOME. It does not correctly add the users to the desired groups. Please check your /etc/group file to see whether the desired users are really in the lpadmin group. If not, add them by editing the file manually or using another user management tool.
CUPS and system- config- printer (gnome-cups-manager is obsolete) are working correctly. All users in the lpadmin group can do printer administration tasks without further authentication. In the CUPS web interface user name and password have to be from any user in the lpadmin group.
Moving the bug to GNOME's user management tool.