Attempted this quest with two friends about seven times, same issue. We switched back and forth who started the instance and I’ve noticed that for the most part it’s whoever initiates the quest that disconnects. Not really helpful info for players, but maybe a small help to AN so they can fix this problem.
Edit: Also, there doesn’t appear to be a specific point at which it disconnected us. One of us disconnected during what i think was the last fight, then again just after Trahearne summons his minions, then once even on the loading screen. The only pattern i can see is that the more often we attempted it, the earlier on in the quest we’d disconnect.