Comment # 14 on bug 991383 from Stefan Dirsch

Oh well. After uninstallation of nvidia drivers and reboot X and lightdm should
have worked. Although with nouveau instead of nvidia ...

Things should have worked after reinstallation of nvidia drivers and reboot.
but lightdm fails for unknown reasons. Another forced reinstall and restart of
xdm doesn't help either (same failure).

But a simple reoboot after that fails. No idea why.

Werner, are you using some special xorg.conf.d snippet? 

Most likely I need ssh access to your machine and need to retest everything in
order to figure out, what's going on. But since this is your working machine
I'm afraid this is no option.