It just happened again. I print-screened the board state, but I can't figure out how to insert the shot from my PC. It seems to only accept URLs.
My opponent had just flown over for 9 damage with Akroma, Angel of Wrath and Angel's Tomb, leaving me at 10 life. The Game Log (Game # 818497180) indicates it is now my turn, but the Phase Ladder shows the game is still at his Damage phase. Then we got the Subject error message.
Game 1 of this Match DID complete (Match # 253843918 - Game # 818493754). But I am unable to Replay that one, either.
EDIT: I DID get the broken game to start a Replay. It just took about 4 minutes for it to start. I should mention these are Big Deck matches, but that has never caused these type errors before Oct. 25th. It just takes about 8 minutes to come back from Sideboarding, and that didn't start until Ikoria was released. (I blame it on Companions, but I'm not sure why they would slow down the loading of the game.)
I have been paying attention to what cards / actions are in play when this happens, and so far this week, it's never involved the same cards twice. It can happen during combat, at end of turn, when a creature goes to the graveyard... just whenever. But never while a spell is being cast, at least the last 6 or 7 times.
I've not been able to discern any rhyme or reason. Some errors you see with the same card a few times, so you know what's causing it. This does not appear to be one of those.
But I will add it hasn't happened against any other opponents this week, so it is probably something in his deck. We're just not sure how to figure out what.