Since I upgraded to ubuntu 13.10 (from 13.04), it's not always possible to switch users: a black screen and an arrow are displayed when an attempt is made to switch from a logged-in account to another one.
My desktop is Unity. I haven't installed another one. Display manager is lightdm.
The system does not freeze, it's possible to escape by going into a console with Ctrl-alt-F1 and kill the current session to return to the login screen. Still, it's annoying.
I have tried rebooting after removing .Xauthority files, including /var/lib/lightdm/.Xauthority, but the problem comes back after a day or two.
Do you have any idea what's going on ?