By the time I load into this dungeon, the group (and the quest-giving NPC) are already fighting a group of enemies near the entrance. As soon as this fight is over the questgiver loses his quest-giving status (marker vanishes, his dialogue options are removed) thus preventing me from accepting the quest from him in the first place.
This is probably because he follows the party through the dungeon as an NPC, then becomes an enemy at the end.
Either way, it's preventing me from accepting/completing this quest.
I've had this happen three times on two separate characters. Please have him retain his quest-giving status for long enough for people to actually talk to him.