Hey, I got some interesting info from someone in my guild:
I get that message sometimes on my emulator when it takes too long for the battle to load. The battle timer starts at 5 minutes even if it took 3 minutes for the screen to load. But apparently there is a timer for when the battle should end too. If the battle starts too slow, that will be exceeded and it’s like a hard close. You lose all progress
I don't see any reason why it should be exclusive to emulators, but it might happen more frequently on emulators. If there's too big a mismatch between when the server thinks the battle started and when your client actually finishes, it's reasonable that there might be a problem.
Did that battle take a long time to load/start? We might have found the answer, if so.