I haven't looked too deep into it for a permenant fix yet, though that has come up on my system twice and it has to do with ipv6 comparability.
Is there a ticket open on this?
A basic workaround/config that will allow radvd.conf to continue is:
interface eth0 {
AdvSendAdvert on;
MinRtrAdvInterval 3;
MaxRtrAdvInterval 10;
prefix 2001:0db8:0100:f101::/64 {
AdvOnLink on;
AdvAutonomous on;
AdvRouterAddr on;
};
};
So if you place that into the file /etc/radvd.conf - re-run apt-get upgrade it will conclude.