Ubuntu 12.04 is stuck in lightdm login (Unity Greeter)

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http://askubuntu.com – The login and Unity Greeter is working. However, if I try to log-in - in a matter of seconds, it would return to the login screen. This happened when I tried to install a package and I restarted my computer. I also tried to uninstall, install, re-install, upgrade the lightdm, gdm and gnome. I installed/reinstalled Ubuntu Desktop/Cinnamon/Gnome3 as well, to try and see if it would work. But it is still the same and I can see the login-lockscreen. In a matter of seconds, I'll be back to the log-in screen. (HowTos)