Ensure the leader VPN endpoint is reachable by ns7. This is documented here:
https://docs.nethserver.org/projects/ns8/en/latest/system_requirements.html#worker-node-requirements
the API server response contains the leader VPN endpoint: it is a host address with a UDP port number used to set up a Wireguard VPN. The VPN endpoint is configured during the cluster creation procedure. Ensure it is not blocked by other network appliances.
In other words, try to use the leader fqdn ( quasar-2.local.ilogic.gr) not its IP.
I don’t know how much matters in a migration but this looks strange.