Hi ts678
Thank you very much for the advice. I have just found the error. I had deleted the SSH key and pressed the “Test Connection” button. Duplicati then asks if you want to trust the key. After I did this, the backup worked again.
The difference now were the quotes.
Wrong (like suggested by the help message (if i remember right)):
--ssh-fingerprint="ssh-ed25519 32 20:21:c1:b7:e4:e6:1f:e0:f6:03:aa:bb:cc:dd:ee:ff"
Correct (and working):
--ssh-fingerprint=ssh-ed25519 32 20:21:c1:b7:e4:e6:1f:e0:f6:03:aa:bb:cc:dd:ee:ff
Unfortunately, the error message/hint suggests to enter the key with quotes. Which apparently does not work. 
Kind regards
Christian