Comment 16 for bug 152107

I think I've my problem solved.

The 'Unlock' button worked if I just type 'users-admin' in the terminal, (without sudo or gksu in front of the command)

Then I checked my 'users-admin' launcher setting in the alacarte menu editor, the command was 'gksu users-admin', so I simply delete the 'gksu', and it worked properly without 'gksu'

I guess it could be caused by my previous installation of 8.04 Release Candidate version, it left the old launcher setting in my home directory, so even I made a clean install of the formal 8.04 release later, the menu still uses the old setting (gksu users-admin) of my home directory.