When configuring Remote Archive, user cannot connect to the remote server because the SSH public key of the remote server is not trusted (its fingerprint is not listed yet in local VyOS' ~/.ssh/known_hosts file).
So we have just documented this workaround in order to temporarily solve the problem:
(currently not merged yet)
But it would be good to properly solve it. Maybe by making VyOS show the fingerprint of the remote host and interactively ask the user whether they trust it or not.