I've seen this happen on a couple of run-throughs now, but haven't been able to pin down the exact cause:
At the Seeker camp by the docks, you have the conversation with Gareth where he tells you that he needs you to attack the Bishop while he raids the ship. That goes as normal, but afterwards he just stays where he is. You can go and fight the final battle (the two warriors that he promises you don't come help) and then afterwards he's still in the same spot asking if you're ready to attack.
I suspect that this situation could be a result of having the dragon knight kill all the Shriekers before Gareth arrived (so the first part of the conversation never even happens).
