When setting up the DNS Forwarding service, I was not allowed to put an IPv6 address for the name-server.
I had to work around the problem by dropping back to an IPv4 address for the name-server:
dns {
forwarding { cache-size 1000 domain localdomain { server 127.0.0.1 } listen-on lo listen-on eth1.5 listen-on eth1.2 listen-on eth0 name-server 8.8.8.8 name-server 8.8.4.4 }
}