Comment 8 for bug 1473149

I was having the same issue. I discovered the issue when I found errors written to `/var/log/seat0-greeter.log`:

    Error writing X authority: Failed to open X authority /var/lib/lightdm/.Xauthority: No such file or directory

Indeed, I only had `/var/lib/lightdm-data`. Running the command

    mkdir /var/lib/lightdm

and rebooting fixed the issue for me.