Create a symlink from /dev/dvd to /dev/sr2

[Markus]

Let's try a different approach and hope that knoppix doesn't notice it, if /dev/cdrom is being recreated all the time, try naming it /dev/cddrive instead.
Comment out this line in fstab: /dev/cdrom /cdrom iso9660 defaults,ro,users,noexec,noauto,uid=knoppix,gid=kn oppix 0 0
Add this line to it: /dev/cddrive /cdrom iso9660 defaults,ro,users,noexec,noauto,uid=knoppix,gid=kn oppix 0 0

Then do:
ln -s /dev/scd0 /dev/cddrive
chgrp cdrom /dev/cddrive

Now, hopefully you'll have the symlink intact after a reboot, check with:
ll /dev/cddrive