Thanx for your reply Oriphus.
Yes I do have a mod chip in the PS2 but I think the problem isn't the PS2, I think the problem is the way the backup is burned.
I think that, when I use Nero or Clone CD, something goes wrong during reading or writing of the DVD. I normally make an image on HD and after that I burn the backup DVD.
When I compare the bytes from the original with the backup there's also some difference in it so maybe I "miss" something on the backup DVD but I choose DVD copy so what else can I do wrong?
I hope someone can explain to me how to make a real good working backup copy of PS2 games because they are very expensive and my youngest son already destroyed one game