This is a known problem in Knoppix.
Refer to /mnt-system/KNOPPIX/knoppix-cheatcodes.txt:
If you would like to edit your X-Server configuration manually
(config file /etc/X11/xorg.conf), use "knoppix 2" to boot
into runlevel 2 (textmode only) and, after changing the X
configuration, start the X environment with "init 5". Note that
you can always leave the graphical environment with "init 2", and
restart it later with "init 5".