I ran this last night and think I know what the problem is. The dialogue box when he comes out of his corridors pops up whilst you're still fighting the last spawn of NPCs. If you minimise the pop up, which you will by using the mouse or activating a skill on the bar then the box minimises but doesn't reappear when you exit combat. I had to leave the instance and redo the mess hall section in a new instance, but on the second attempt left my BOs to fight the spawns in the corridor so that I was out of combat when the pop up appeared on screen so I could press continue. Doing it this way I managed to get the mission to progress to the next section.