Ok, got it! It was really the bootloader - I bought two shields and tried with both - same error. Now I flashed the bootloader on one of them - and an...

I meant this picture.

@mario Hi,I tried nearly everything (baudrate, ...) from the mentioned post - and the arduino is not responding.You wrote how to flash the bootloader,...

@mario Hi,I retried the soldering, but got all the time the same message in the logfile: 2020-01-29 21:00:44,195 - fabscan.server.FSScanServer - INFO ...

@mario Thanks for the fast answer - I'll check this - good to know, which are the important pins! Christoph