There is community thread and an explanation in the Apple Knowlege Base about the broken IPv6 tunnels in the new 7.6.3 version for Time Capsule/Airport devices. But the Native IPv6 is also broken on this version. I also needed to rollback to version 7.6.1.
The weird thing is that the configuration somehow ignored your settings and changed to the 6to4 prefix (described in RFC 3056).
Any forecast of when Apple will update this IPv6 broken release?
I wrote a some topic on my blog if you need more details.