****Host Key has changed.****
I assumed that the public keys in each server’s /etc/ssh/ssh_host_rsa_key.pub files (and the matching private keys) are the actual master keys.
I copied the public keys from each other node into both /etc/ssh/ssh_known_hosts on all three servers, and /etc/pve/priv/known_hosts is symlinked which is shared across all three.
Once that was done, the replication errors went away and the cluster appears to be operating normally.
Symlink wie folgt:
ln -s /etc/pve/priv/known_hosts /etc/ssh/ssh_known_hosts