I might have solved the issue with the "empty IP list". In my case, I was only able to add a server to the backup infrastructure by its IP address, not with the FQDN. I found, that the DNS(A) host entry for that server was incomplete (IPv6 entry was present, but IPv4 wasn't). After adding the IPv4 entry manually, I was able to add the server to our existing Hyper-V cluster configuration in Veeam as another cluster knot, using the FQDN. I started a test backup and everything seems to be working now.
Hope this helps to others.