It's still happening on 27.0.1 since a number of users on that version have reported the issue to us. I haven't tested on the 27.2 beta.

Thanks for your detailed detective work! It's very useful. We also have a game with a two player real-time GameKit mode. We've gotten reports from many users that the game is crashing on iOS 27 when playing a real-time match. Looking at a crash dump, the stack trace is identical to the one posted by you: CFRetain GCKSessionReceiveDOOB -[GKSessionInternal receiveDOOB:fromPeer:inSession:context:] -[GKSessionInternal(_private) tellDelegate_didReceiveBand_RetryICE:] __NSThreadPerformPerform __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ We tested on iOS27 ourselves, got a crash in the second match, and the crash happened in the 14-21 second window after the second match started. So the symptoms are identical to the ones you describe. Hopefully, Apple will fix this soon, as real-time matches are unplayable right now.

It's working now for me (June 8, 2022) with no changes on my end.

I'm also having the same problem and getting the exact same error message today (June 7, 2022). It was working a few days ago, but it doesn't work now. I'm using a Windows 10 machine with Java 1.8.