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I have completed all suggested updates for 12.04 LTS after a long time of inactivity with my administrative account as the old laptop (Sony Vaio) was used by my wife only. After the update the Unity was gone (and a message like "Compiz was closed unexpectedly"). Trying every solution I found was not helpful. Eventually I upgraded to 14.04 LTS (by do-release-upgrade). When upgraded and rebooted, after logging in I get a pop-up "System problem detected". After a few seconds it comes back to login screen, and that's true for all users including guest user. I tried every suggested solution I found including changing lightdm to gdm to no avail. Again removed the Gnome desktop, problem persists. Would somebody please help to find a solution? I appreciate it.