Comment 2 for bug 1021398

ifupdown works sequentially, so in this case you're asking it to configure an ipv6 address on an interface you haven't defined yet.

Move the "iface br0 inet6 auto" after the "iface br0 inet static" section, that should fix it.

Also, I don't think you actually need that line at all as the kernel's default behaviour is to accept router advertisement.

Marking the bug invalid as that syntax is indeed invalid. Fell free to move it back to new if the bug still happens after fixing the ordering.