I have never had my game disconnect in a single-player quest, yet it happens in 70% of my multiplayer games.
Here is the thing, I am using LAN settings since even though I opened ports 1000-1005 and configured my firewall settings for the same, I cannot connect to a Co-op game using WAN settings.
My buddy and I pretty much spend half of our playing time trying to fix this issue and end up sitting side by side playing quests individually.
Are there plans to change Co-op issues in future updates? Anyone facing similar issues and find a fix?