Four sepulcher bug

hello, i encounter this bug from time to time. when i finish four sepulcher, kill last boss and teleport to area with a ghost that is supposed to open them boxes with loot, he sometimes stop doing that in the middle, for example i manage to open 400 boxes and got like 800 left and he says something like "you dont have a quest for this npc or do not meet criteria". just to clarify, i still got the four goblets quest and i'm not exceeding 80% weight limit. its just random, sometimes i open them all, sometimes it wont let me open them at all. problem is always fixed the next time i go through whole 4s, kill boss again and so on. any ideas?