This was what worked for me
sorry to necro this post but this definitely worked for me and I never thought I would be able to complete it.
I was stuck on this also. I’m not sure however how far you have to go to complete the zereth mortis story. So I ended up after lots of attempts, go to bastion drop and restart the quest, fly to the chapel which made it transition to uther, then back out of the room, which sent me to bastion, then clicked the door/thing in bastion and it worked.
I’m horde and just completed it by disabling all addons. It had bugged on me 3 times before trying this. Hopefully it helps some of you.