Finally found a work-around that also proves the failure to become configured is caused by that stupid ipv6ll address added to the DNS server list. But I did not find any way which integrates into netplan. I basically took /run/systemd/network/10-netplan-br0.network and copied it as /etc/systemd/network/05-br0.network.
Then added: "IPv6AcceptRA=no" to the [Network] section and rebooted. Now the DNS list only contains the one server I specify and the status ends up as "routable (configured)".
Finally found a work-around that also proves the failure to become configured is caused by that stupid ipv6ll address added to the DNS server list. But I did not find any way which integrates into netplan. I basically took /run/systemd/ network/ 10-netplan- br0.network and copied it as /etc/systemd/ network/ 05-br0. network.
Then added: "IPv6AcceptRA=no" to the [Network] section and rebooted. Now the DNS list only contains the one server I specify and the status ends up as "routable (configured)".