So, I started to dig a bit deeper by simply downgrading the qmeu-utils package.
Running the recent version of buster but version qemu-utils_2.8+dfsg-6+deb9u8 (http://snapshot.debian.org/package/qemu/1%3A2.8%2Bdfsg-6%2Bdeb9u8/#qemu-utils_1:3a:2.8:2b:dfsg-6:2b:deb9u8) works. All other versions greater than this one did not succeed. But with qemu-utils_2.8+dfsg-6+deb9u8 all works again, as expected. I hope this helps. Cheers, Marcel
