Had a rather frustrating bug during the Ulduar scenario - it took me a couple of hours to figure out what the cause was and I *think* I know what is happening so I submitted a bug report and I'm posting this to hopefully save people time and a lot of frustration.
Towards the end of the scenario during Stage 6/8 "A Feat of Battle" I was killing Blood Queen Morvaniss only to have the door after her leading to Thorim stay resolutely shut. If you kill her and the door stays shut that's it, scenario is bugged and nothing can be done about it except to drop the quest and try again.
The spider mobs in the gauntlet during Stage 6 hit pretty fucking hard and the only way I was staying alive as arms was by kiting them when on low health and healing up with Second Wind. While kiting them I noticed that there were spiders in the background who had dropped down which I didn't aggro (would not have survived pulling them) that then despawned. The next time I ran through the scenario I re-specced to Prot, made sure I aggroed every arachnid turd that dropped from the ceiling and the door opened first time after killing Morvaniss.
So while I can't be 100% sure that was the cause of the bug I've got good reason to believe it. Hopefully this helps.
TLDR: Do the Ulduar scenario as Prot.