Hmm, had a look at my Kanotix BHX install and it gives the same stuff at bootup:
mount: fs type devfs not supported by kernel
umount: devfs not mounted

It doesn't bother the bootup though and I rarely have a look at the bootprocess as the box is basically a blind ssh server for me so I haven't even noticed this before.

If you have a look at /etc/default/initrd-tools.sh and /etc/init.d/checkroot.sh you'll see that devfs is used by initrd and checking if the partitions are clean. The only solution that comes to mind is using udev instead of devfs so you might want to have a look at udev also. Kano is making the next BH version so you could also ask in #kanotix if there's any chance of udev making it's way in there.

BTW, a reason for reboot helping could be that the partions aren't checked then as they are at a normal bootup. Not sure about this one though, just a thought.