Maybe a bad burn? Have you tried to boot with cheat code "testcd"?
I have just downloaded the knoppix DVD, DE version, it took me long time, and this is the surprise booting from DVD:
Accesing KNOPPIX CDROM at /dev/hdb... insmod: error inserting '/modules/cloop.o': -1 Bad file number
This is an annoying error after all this download time!
The md5sum is ok, checked on the News Forum.
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Maybe a bad burn? Have you tried to boot with cheat code "testcd"?
You have a bad DVD media or it was burned too fast. Fortunately we have a downloading & burning frequently asked questions which will help you burn the ISO correctly, http://www.knoppix.net/wiki/Downloading_FAQ. You will find the advice you need in there.

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