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.