Mission 2 Coop Campaign Crash

Hey,
I am new to Halo Wars 2, I play the campaign in coop mode with a friend, in mission 1 everything went fine, in mission 2 “a new enemy” the game crashes to desktop at my computer. After about 10 seconds the connection fails and my friend is forced to leave the mission as well. We played it 5 times and always tried to change something in game settings or use windows mode or reinstall the game, but it still crashes. I think it happens when there ist new content like picking up items or the artillery coming on the map, or the fight at the end of the map.
We played skirmish 2v2computers for about 45 min, there was no crash and we could finish the match.
System information:
CPU: Intel Core i5-4690k
8gb ram
GPU: amd radeon r9 200series
OS: windows 10 version 1803 build 17134.81
Thank you