Problem
You receive a notification from the Apple Resolution Center (= the team at Apple iTunes that reviews your submitted app) with the following message:
We discovered one or more bugs in your app when reviewed on iPhone running iOS 9.3.3 on Wi-Fi connected to an IPv6 network.
Cause
The cause of this is a temporary network problem at the Apple Resolution Center.
Solution
Resubmitting the same .ipa solves the problem.