After following the usual steps (a search for ssh login no password will give plenty of tutorials) I was still unable to login to my second server (which runs ubuntu 22.04).
At first I thought the problem was that RSA was deprecated. So I switched to ecdsa
ssh-keygen -t ecdsa
That did not allow me to login either.
However, when I debugged the login in with:
ssh firstname.lastname@example.org -vv
I noticed that ssh was looking for
~/.ssh for the private key. So I copied the file with the private key to
id_ecdsa and was able to login subsequently.
I was under the impression that the file with the private key on the local computer could have any name. Perhaps that assumption was wrong.