Hi guys,
that worked perfectly, thanks a lot!
I bought an Athlon 200GE, installed it and got into the BIOS immediately. The installed version was 4207. I downloaded version 5204 from the ASUS site and installed that version. I re-installed the Ryzen CPU again, it booted without any problem. Again, many thanks, especially for telling me the cheapest CPU that runs on the old version.