I will try to collect.

In the mean time I've found that SSH could be in some way involved

When I add the host and get the immediate failure and apparently nothing happens at all,  I see these two lines in /var/log/ovirt-engine/server.log

2020-10-09 18:15:09,369+02 WARN  [org.apache.sshd.client.session.ClientConnectionService] (sshd-SshClient[7cb54873]-nio2-thread-1) globalRequest(ClientConnectionService[ClientSessionImpl[root@ov200/10.4.192.32:22]])[hostkeys-00@openssh.com, want-reply=false] failed (SshException) to process: EdDSA provider not supported
2020-10-09 18:15:09,699+02 WARN  [org.apache.sshd.client.session.ClientConnectionService] (sshd-SshClient[2cbceeab]-nio2-thread-1) globalRequest(ClientConnectionService[ClientSessionImpl[root@ov200/10.4.192.32:22]])[hostkeys-00@openssh.com, want-reply=false] failed (SshException) to process: EdDSA provider not supported

could it be that the ssh client embedded is not able to connect to the CentOS 8.2 for some reason?

On host at the moment when I try to add it I see again two sessions opened and immediately closed (tried several times), eg in the timeframe above I have:

Oct  9 18:15:09 ov200 systemd-logind[1237]: New session 41 of user root.
Oct  9 18:15:09 ov200 systemd[1]: Started Session 41 of user root.
Oct  9 18:15:09 ov200 systemd-logind[1237]: Session 41 logged out. Waiting for processes to exit.
Oct  9 18:15:09 ov200 systemd-logind[1237]: Removed session 41.
Oct  9 18:15:09 ov200 systemd-logind[1237]: New session 42 of user root.
Oct  9 18:15:09 ov200 systemd[1]: Started Session 42 of user root.
Oct  9 18:15:09 ov200 systemd-logind[1237]: Session 42 logged out. Waiting for processes to exit.
Oct  9 18:15:09 ov200 systemd-logind[1237]: Removed session 42.

anyway at sshd service level it seems it is ok om the host:

journalctl -u sshd.service has

Oct 09 18:15:09 ov200 sshd[13379]: Accepted password for root from 10.4.192.43 port 46008 ssh2
Oct 09 18:15:09 ov200 sshd[13379]: pam_unix(sshd:session): session opened for user root by (uid=0)
Oct 09 18:15:09 ov200 sshd[13379]: pam_unix(sshd:session): session closed for user root
Oct 09 18:15:09 ov200 sshd[13398]: Accepted password for root from 10.4.192.43 port 46014 ssh2
Oct 09 18:15:09 ov200 sshd[13398]: pam_unix(sshd:session): session opened for user root by (uid=0)
Oct 09 18:15:09 ov200 sshd[13398]: pam_unix(sshd:session): session closed for user root

On the host I have not customized anything ssh related:

[root@ov200 ssh]# ps -ef|grep sshd
root        1274       1  0 Oct08 ?        00:00:00 /usr/sbin/sshd -D -oCiphers=aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes256-cbc,aes128-gcm@openssh.com,aes128-ctr,aes128-cbc -oMACs=hmac-sha2-256-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha2-256,hmac-sha1,umac-128@openssh.com,hmac-sha2-512 -oGSSAPIKexAlgorithms=gss-gex-sha1-,gss-group14-sha1- -oKexAlgorithms=curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1 -oHostKeyAlgorithms=rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384,ecdsa-sha2-nistp384-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com,ecdsa-sha2-nistp521,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,ssh-rsa,ssh-rsa-cert-v01@openssh.com -oPubkeyAcceptedKeyTypes=rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384,ecdsa-sha2-nistp384-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com,ecdsa-sha2-nistp521,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,ssh-rsa,ssh-rsa-cert-v01@openssh.com -oCASignatureAlgorithms=rsa-sha2-256,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,rsa-sha2-512,ecdsa-sha2-nistp521,ssh-ed25519,ssh-rsa

and in sshd_config