Neither lightdm not gdm actually allow me to log into any account but guest. My solution, so far, has been to drop to the shell (ctrl+alt+f1) login, then sudo startx. the xauthority file never locks without sudo.
Doing it this way, I'm root. I have not been able to successfully access a non-root terminal from root. As a consequence when I want to run some file or program which objects to being run as root, I have to switch to the guest session.
I really can't believe how broke this release was, the same thing happened on 2 computers. I don't so much mind going around the normal login, however the unwanted root access this creates is a major issue.
edit: after further investigation I found that su username was exactly what i was looking for to de-root a root terminal