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I have a computer, I think it's running GNOME (it's running whatever the default is; I have the Unity panel,) and I installed LXDE for use on another user, but after installing LXDE, it will no longer lock the screen due to inactivity, or for any other reason.

When logging in, I have to enter my password, just like normal, but the screen cannot lock. Wait 5 mins (my setting, have verified this,) still unlocked. Press CTRL-ALT-L, no response. Even going to the gear and choosing "Lock Screen" has no result.

Even after deleting the second user account (just on a hunch,) the screen STILL cannot lock.

What do I need to do?

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Have you tried removing xscreensaver? – Uri Herrera Jan 14 '13 at 22:32
Try running sudo dpkg-reconfigure lightdm and selecting lightdm then restarting. Does that help the issue? – Seth Jan 14 '13 at 23:16
Aha. sudo apt-get install gnome-screensaver fixed it. (I want to keep XScreensaver installed, as that's the only working option for LXDE.) Now I just have to figure out what to add to the startup file: gnome-screensaver --display=? I've tried numbers, but that doesn't work. EDIT: @iSeth, I just saw your answer, and tried that. So far, logged into the primary account, all is fine - will reply with info after logging into the LXDE account. – JamesTheAwesomeDude Jan 15 '13 at 0:42
YES! Both users can now be locked without issue. – JamesTheAwesomeDude Jan 15 '13 at 0:48

Okay, first, be sure GNOME Screensaver is installed (it's not really a screensaver manager, but more of just a daemon to handle locking/unlocking the screen):

sudo apt-get install gnome-screensaver

Then, reconfigure LightDM to incorporate the changes (credit to iSeth for the original suggestion,):

sudo dpkg-reconfigure lightdm

Finally, reboot. Everything will be fixed! Both LXDE and Unity/GNOME can now be locked without problems!

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