I was aware, that COP tool itself works correct, as mentioned in my post.
I want to unpack/convert via XAD!
Thanks anyway.

Then doesn't that suggest XAD simply can't handle COP files!

Meanwhile i have found this:
http://www.dstoecker.eu/xadclients.html#Clients

And some formats, which need to be expanded.
COP!addition of missing algorithms

So that explains why XAD can't cope with all COP files! Strange I got to track 72 though? COP also seems to write the bootblock/directory after it's wrote all 80 tracks to disk, hence all disks seem corrupted when they fail in XAD!

Ok, solved.:)
I have to use an older COP client.

OK folks, everything is much simpler.
First, there is the source to the client on aminet (version 1.3) which ironically comes with the current XAD distribution, so it can be fixed, no idea what's wrong in it, possibly a compiler problem.
Secondly, there is an older client v1.2 and it works perfectly!
No idea what really changed in 1.3 that makes it fail.