One thing that fixed it for me was doing three secrets in the Trenchway, the Mausoleum, and the Hall of Souls. For the Trenchway there is a door with a bunch of taken orbs all over it with two torches to light on either side and another one on the higher platform directly across from it. Lighting all three spawns an enemy to kill then loot its chest. In the Hall of Souls you will find a boss cursed thrall eating some worms between one of the two rooms. Do not let him explode and lead him into the room next to him where another pile of worms are. He will eat those and transform into an acolyte. Then take it out the other side to a corner with even more worms that will turn him into a knight where you can then kill him and loot the chest. In the Mausoleum down by the boss area are three little taken blights that are suspending dead cabal in the air attached to tendrils. Two are easily seen and the third is tucked away in a corner to the back left. A fourth one is found slightly up the hill that usually have hive and taken fighting in the area. Once the all four are shot down and the spawned taken are dead a third chest will spawn. I was missing the one in the Hall of Souls and as soon as I did it gave me an item and was gave me access to the panel you have to go to. https://www.youtube.com/watch?v=xQ7iszfNWFg This video is what I used.