I now have a functional VM running Ubuntu 16.04LTS and BackupPC 4.0 (from the master) source.

I have "su - backuppc -s /bin/sh" and acquired the "$" prompt.

Generated the keys with "ssh-keygen".

Now when I:

$ ssh-copy-id root at client ipaddress    <<< sanitized for security
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/var/lib/backuppc/.ssh/id_rsa.pub"
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
Permission denied (publickey,password).

I have not had a chance to search why, yet.

This works fine with a repo install of BackupPC v3.3.1 on Ubuntu (has been my experience) so, my guess is the repo package dependencies takes care of what is causing this. I know that by default Ubuntu does not allow root ssh access therefore it leads me to believe that the "packaging" sets the ssh access rights to allow the "copy" to work.

How is the old version (v3.3.1) getting around this by not manually adjusting the /etc/ssh/ssh_config of each client to allow root logins?

So, to potentially help someone else with the same problem, I pose the question here. What do I do to correct this?