Comment 10 for bug 1087920

As I just posted on #1048657 , I had this crash bug in 2.6 and 2.8. It had apparently been caused by an inadvertent substitution of my .X display manager. This substitution also caused shell scripts running as root (on system startup) to fail .X related tasks due to a bad MIT magic cookie and and a lack of proper authorization (gdm was changing the XAUTHORITY cookie directory for my main login at every reboot, so the root user couldn't link to that cookie)

In any case
sudo apt-get install --reinstall lightdm unity-greeter
rm ~/.Xresources
rm ~/.Xauthority #without this (and possibly the one above) you may have difficulty logging in to your primary user account
sudo dpkg-reconfigure lightdm #select lightdm, rather than gdm, etc
echo -e "/usr/sbin/lightdm" | sudo tee /etc/X11/default-display-manager #this may have been redundant