Dear all.
thanks a lot for the supporl.
It was actually some wrong permissions for the folder .ssh/ and the file authorized_keys in combination with the lack of restarting the ssh-server. Sorry for that.

Now it works. The solution is as shown in https://docs.ubports.com/en/latest/userguide/advanceduse/ssh.html

chmod 700 /home/phablet/.ssh
chmod 600 /home/phablet/.ssh/authorized_keys
chown -R phablet:phablet /home/phablet/.ssh

and after this modification, stop and restart of the ssh-server
sudo servcie ssh stop
sudo servcie ssh start
systemctl status ssh

Thanks a lot
Jörg