My ubuntu machine lost IPv6 connectivity beyond the link-local network. It turned out to be the same problem someone described at askubuntu. According to a message in the ubuntu-bugs mailing list this should be resolved by an update soon.

Until then, announcing a specific hop-limit will work around the problem

/ipv6 nd
set [ find default=yes ] advertise-dns=yes hop-limit=64 \
ra-interval=20s-1m ra-lifetime=3m