OK from what you have described, it seems the problem is due to knoppix-data.img is grown to totally filled up and it makes KNOPPIX un-fit for the next boot. Not sure how much knoppix can do about this but perhaps a check can be done before shutdown.

Nevertheless, for the more proficient users, this problem can be fixed by booting into 'debug' shell, loop mount knoppix-data.img and free up space. It should not need a re-wipe to clean up everything :-

1. Add the cheatcode 'debug' into syslinux.cfg.
2. Boot the computer.
3. At the shell prompt :-
# mkdir /tmp/tmp
# mount -o loop=/dev/loop7 /mnt-system/KNOPPIX/knoppix-data.img /tmp/tmp
# cd /tmp/tmp
..... ( start to free up space below this directory, be-warned not to remove your important files )
# cd /
# umount -d /tmp/tmp
4. After that type 'exit' 6 times.
You will be booting correctly if you have freed up enough space.
5. Remove cheatcode 'debug' from syslinux.cfg,