I was finally able to pick up the Vizier’s rank 4 quest “Knives in the Dark” a little bit ago. However, after completing the objectives, the quest marker on the map and minimap direct me inside a random building in the Umbral Bazaar directly to the north of the Royal Apothecary and there is no NPC or object to turn in to. Returning to the Vizier (Executor Nizrek) where I originally picked up the quest also does not allow turning in.
I additionally checked his representative in the Weaver’s Lair just in case, but it sadly also cannot be turned in there. I also don’t have any other active quests in the entire zone.
EDIT: Nevermind, this quest is apparently part of rank 7 and the rank 4 quest is still bugged.