Can't login after using Ubuntu Tweak
It happened to me too. use gdm to replace lightdm
- go to a text terminal using alt-ctrl-F1
- Stop LightDM with sudo stop lightdm
- Start GDM with sudo start gdm
- Run sudo dpkg-reconfigure lightdm to set the default display manager for gdm
- Edit /etc/X11/default-display-manager and set it to /usr/sbin/gdm if you can't run the above
- Restart your computer and login.