Problem solved it was the sshd_config Authentication section
On the Machine you want to install k3s via k3sup install --ip --user
root --cluster you must configure passwordless root Authentication like this:
Authentication
PasswordAuthentication no
PermitRootLogin without-password
then systemctl restart ssh.service
On the Master node from where you rum k3sup install as root user (important) leave the sshd_config untouched!!!
Before editing the sshd_config file on the new master-node you have to ssh-copy your rsa ssh_key (ed25519 does not work) to the new Master node like eg ssh-copy-id root@ip-address
Thanks so much for all your help.
Uli