Ah, I still had one of those bad archives left.
So I could see that the file size differed.
The bad .tar.gz was 182.056 bytes
The good 191.865
And after unzipping the corrupt file I got a .tar file of size 191.865 !
Well, I renamed the unzipped corrupt .tar file to .tar.gz
And now things worked well.
unzipping THIS file resulted in a .tar file that could be extracted.
So - however it happend - the "corrupt" file had been gzipped twice.
At least a partly explanation to this mystery :)