Comment # 7 on bug 1178812 from Stakanov Schufter

More precise information:
So the terminals 1 to 6 are functional. 
Terminal 7 is stable, but once the user on TTY8 is opened and you try to change
back the terminal to TTY7, you see a kind of white flash shortly on the screen.
Then you are at the first user. When trying to go back, you find a login screen
on TTY8 that is not functional but the user is now on TTY9 (functional). This
is stable. All terminal above TTY9 are without function or output. 
So I think something crashes when trying to change user.