Xserver closes after logging in

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http://askubuntu.com – Recently I've added ppa:xorg-edgers to software sources and upgraded all packages. After I restarted and logged in the screen flashed and closed xserver and the same thing happened when I typed startx in terminal. Lightdm works correctly but xserver encounter some problems so that I purged the previous ppa using ppa-purge xorg-edgersto rollback changes , reinstalling or reconfiguring xserver but it didn't work. This is my log file /var/log/Xorg.1 : [ 2913.764] X.Org X Server 1.13.0 Release Date: 2012-09-05 [ 2913.764] X Protocol Version 11, Revision 0 [ 2913.764] Build Operating System: L (HowTos)