This is a problem it seems that if you don't have your guest yet installed on a virtual drive and you re-boot from within the guest using its logoff you end up
crashing. this is with virtualbox 1.3.8 and Sidux 2.6.20.7 it would be nice if it rebooted the .iso that it started from.
In my case I used the drop down Devices and selected the cd .iso to boot
from to start my session.