I've just looked at Apple's documentation about authenticateWithCompletionHandler:. It says it's deprecated in iOS 6.0 but also it should work without problem. I'll look into it.
I've tested and it worked without any problem on iOS 7.0.2 (real device), iOS 6 (simulator) and iOS 7 (simulator).
code=2 means "The requested operation has been cancelled.". If you have cancelled the login window a couple of times, Game Center doesn't ask again and automatically gives this error. You may need to double check your sandbox account.
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code=2 means "The requested operation has been cancelled.". If you have cancelled the login window a couple of times, Game Center doesn't ask again and automatically gives this error. You may need to double check your sandbox account.