@orindine I also have this issue on Xbox one. A cave supposedly holding a standard loot chest behind a waterfall near anlafs lookout while playing the War in the North mission. Once entering the cave, the path is blacked out and Eivor begins swimming in the air. As he is 'swimming' he cannot progress further into or even exit the cave. Can only be reversed by restarting game/console, loading a previous save or fast travelling. Problem still occurs making the loot chest impossible to access unless there is a second path that I don't know about yet.