Comment 6 for bug 856810

I had the exact same problem and did the steps in comment #3 with the exact same results.

So, I tried:

sudo dpkg-reconfigure lightdm

That appeared to resolve the problem once I switched the default display manager back to lightdm.

After that I was able to do:

sudo apt-get purge gdm

And I did not have any problems getting to the login screen with lightdm after that.