Just played a coop match with a friend and also had this problem. Putting "guild involvement" or how it's called dropdown to "little" already, after playing a few hours, we suddenly have to kill all "Shadow" in one hour and twenty minutes, a race we did not, and after explore-rushing a full hour still did not see. At this point we thought we had all sectors or how it's called explored (at least the gates leading to each). After twenty further minutes, it was over. A 5 hour beginner match of a supposedly open ended game rudely stopped by some "well, tough luck pal" message.
Please make dropdown "guild involvement" to select: NONE.
While you're at it, make an option to continue the game even if you did not "win", i.e. endless play.