Apparently not only the permissions on the .ssh directory but also the user directory (on the remote host) should be set correctly. In my case to 766 (perhaps different on other configurations?). For some reason this was set to 776, which didn't work after all the above. After a "chmod 766 /home/$USER", I could finally login with ssh for this user using the key authentication. Although the ssh -v is also useful for debugging, journalctl showed this line: "Authentication refused: bad ownership or modes for directory /home/username", which brought me to the idea to compare my directory permissions to different user directories that did accept a no-password ssh connection. This was on a MX-Linux (Debian based) laptop, February 2025.