I see the error message say, "indexing nil value when runnnig the menuConfirm command".
I'm pretty sure the lobby somehow reset internally on your side for some reason and your opponent was still connected to your lobby and tried to press the "ready up" button on their side and it got to you and caused a crash cause there player data no longer existed on your side making it a nil reference.

I found the cause of the lobby reset bug. it was a stray line I missed in the map selection mini-menu when you hit the back button to close the menu it ticked the flag to reset the lobby when you return from in game. 
My bad :^(

now it makes alot of sense why this occurred when you decided to play on a different  map

alright the patch has been pushed out