Could you try to
- Install lightdm-gtk-greeter through apt
sudo apt-get install lightdm-gtk-greeter
- Open up ‘/etc/lightdm/lightdm.conf.d/50-nvidia.conf’ with any editor as root.
Add below line.
greeter-session=lightdm-gtk-greeter
Actually, this is the final solution we have here. Next release will contain this too.