How can I prevent myself from getting logged out automatically in a multi-user setup?
You can check that you have enough TTY's:
Open a terminal and do:
grep tty /etc/default/console-setup
It should show (check the last number, it should say 6):
ACTIVE_CONSOLES="/dev/tty[1-6]"
If not, edit the /etc/default/console-setup file by typing:
gksudo gedit /etc/default/console-setup
And look for ACTIVE_CONSOLES, and make the line look this:
ACTIVE_CONSOLES="/dev/tty[1-6]"
In the end I lived with this for a while and ended up upgrading Ubuntu to a newer version and it went away. So not sure what it was but probably a bug somewhere..