Comment 7 for bug 1234400

I've just faced this problem (removing .Xauthority solved it).

I've tested again with an invalid .Xauthority, and the login failed.
I then installed the proposed lightdm version (1.6.0-0ubuntu4) and restarted the service.
Tried to login again, and it seemed to work!

It's probably better for someone else to verify this.