>> What could be wrong?
> 
> Problem with isolinux 4, I'm investigating.
> I just uploaded grml-live 0.9.43 to grml-testing which addresses this
> issue, please let me know whether it works for you as well.

Yep,

rm /dev/shm/gl/grml_cd/boot/isolinux/ -rf
rm /dev/shm/gl/grml_isos/depen64.iso
grml-live ... 

problem solved. CD boots fine, having used grml-live 0.9.43.