Our update has been rejected twice today for ipv6 network connectivity issues. Our networking code has not changed between the previous release and this current release.
We ran into this same problem, and it turned out while we had setup an AAAA record for IPv6, since we didn't actually have IPv6 support (we're also using Route53), it borked everything. Removing the AAAA record fixed the issue.
I've filed a radar about the discrepancy between the documentation for testing and the setup App Review is using - we were only able to diagnose it because our CTO was at WWDC and was able to connect to their network, which is not exactly a situation we can reproduce regularly.