Because of bug 933640 (https://bugs.launchpad.net/nova/+bug/933640) hairpin mode is now enabled by default on all bridge ports. When using IPv6 an instance sees its own neighbour advertisement, because of the reflective property of the hairpin mode. Because of this the trigger-happy duplicate address detection in the instance's kernel deconfigures the IPv6 address on the interface, resulting in no IPv6 connectivity.
router advertisement recieved -> configures IPv6 on interface-> sends neighbour advertisement -> receives own neighbour advertisement -> removes IPv6 address from interface
from instance' hum' syslog: Jun 10 10:19:44 hum kernel: [ 150.028370] eth0: IPv6 duplicate address 2001:6f8:1477:1111:f816:3eff:fe20:48ad detected!
after disabling hairpin mode on the compute node of the vnet interface used the error disapeared and IPv6 connectivity was enabled.
I can confirm this problem.