Hey, I'm playing coop with my friend and we both have this problem.
Both on pc win 7, he's the host.
After alt tabbing out of the game, trying to focus back to it when it's your turn in combat crashes that players game. Same problem in compatibility mode and in fullscreen/windowed/borderless. We still love the game and will keep playing, but sometimes turns get a bit long and it's nice to check a webpage every so often. I'll keep looking for a fix but post here if anyone else has this issue.
