system preferences pane not unlocking

Hi everybody,

I wanted to create a new user session on my iMac (10.5.8). To my surprise, when I clicked on the little padlock to allow changes to be made, instead of asking me for the password, it said "Authenticating" and then goes back to "Click the lock to make changes".

I have tried changing the Admin password, I've tried setting no password, I've verified keychain, and I made the mistake of deleting ~/Library/Preferences/com.apple.HIToolbox.plist. That freaked me out as I couldn't access neither my 2 sessions anymore. Deleting that file somehow made my computer forget the sessions passwords!

Then, I was totally stuck, I couldn't do anything form inside a session, so after looking for it on the Internet, I rebooted and clicked Cmd+S to enter Single mode and I could change ONE session password through some scary command lines... but not the other one. Luckily, the session which password I could change is the one from which I formerly deleted that .plist file. Then (because I made a copy of it before deleting it) I put back in the folder and my other session's password came back...

Still, I am still not able to unlock the prefs.

Any idea?

Thanks in advance.