Problem description: an Ubuntu client doesn't configure the IPv6 address at boot time. But running manually "dhclient -6" works. dmesg reports:

[   11.997321] type=1400 audit(1372427393.005:35): apparmor="DENIED" operation="open" parent=1053 profile="/sbin/dhclient" name="/var/lib/NetworkManager/dhclient6-eth0.conf" pid=1765 comm="dhclient" requested_mask="r" denied_mask="r" fsuid=0 ouid=0

Solution: there's a typo in the /etc/apparmor.d/sbin.dhclient file. In the Network manager session of the file, replace /var/lib/NetworkManager/dhclient-*.conf lrw, with /var/lib/NetworkManager/dhclient*.conf lrw, (remove the dash - before the star *).

More information: https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1180516