Workaround (if you're root, not suitable for the guest):
Switch to console (Ctrl-Alt-F1), login as user, su -, passwd guest -> choose a valid password, exit console, switch back to X (Ctrl-Alt-F7).
Workaround (if you're root, not suitable for the guest):
Switch to console (Ctrl-Alt-F1), login as user, su -, passwd guest -> choose a valid password, exit console, switch back to X (Ctrl-Alt-F7).