App rejected because of a Game Center Bug
Ever app developer knows the horror of being rejected by Apple. There are tons of legitimate reasons, but from time to time, it just happens without your fault. It happend to me twice in a row because of a bug in Apple's Game Center.
Fortunately Apple provides you with crash logs. Just download them and drag and drop on the XCode-Icon in your Mac Menu Bar. This symbolicates the crash log and makes it readable to you.
It just says
GC Framework: startAuthenticationForExistingPrimaryPlayer
in the Breadcrumb Trail. It took me a while and two submissions to figure it out. It happens when you are trying to log the user into Game Center and the reviewer on Apple's side is logged into Game Center with a Sandbox account from a different app.
Just ask the reviewer kindly to log out and try again. Easy solution, but possible hours of fun and bug hunting.