Still didn't find anything that could help me solving that issue. And nobody seems to have a idea.
I did some experimenting:
the faster i burn, the more likely it works:
8x and below always fails within the first 10MB
12x get's pretty far, and sometimes eaven finishes.
24x usually finishes.
However - I prefer to burn at a slow speed, so the result is as readable as possible. Nothing worse if you burn a knoppix cd, and the drive slows down because it has troubles reading the cd
I put my burner into my old box, so i can burn the new knoppix cd. Actually i bought the new board because the old one is unstable, but until now it's just a big disappointment