Trying to use Knoppix 7.6 DVD as rescue in Virtualbox

I am doing a linux course. On a Windows 7 PC I have installed Virtualbox. Inside Virtualbox I have CentOS 6.7 and Ubuntu.
Now is the topic rescue cd. So I have downloaded Knoppix 7.6.1. I can boot the pc with the DVD and get what I guess is the grafical UI of knoppix.

However, when I use Virtualbox and try to start the CentOS virtual machine it starts to boot from Knoppix OK but stops with:
Konnte Disk nicht nach /mnt-system mounten. Starte debugging shell:
sh: can't access tty; job control turned off

Can somebody tell me if this is supposed to work or am I trying the impossible?

Thank you and regards,
Mark

Is your virtual machine created as 32 bit ? I run Virtualbox 64 bit on a 64 bit Linux host, and I cannot boot Knoppix on a 32 bit virtual machine, although I specifically choose the 32 bit kernel by wriing "knoppix xkeyboard=dk" when booting. Adding no3d makes no difference. It hangs at "Booting the kernel". On a 64 bit virtual machine I can boot Knoppix 32 bit and 64 bit without problems, even without no3d.