I am currently playing as a necromancer and some time ago i received a second cube from Cain, strangely enough. ( i think it was that i loaded my character and my original cube was gone, so i talked to cain and he gave me two cubes instead) Seeing that i could use both without any trouble i made my way to Hell difficulty, although now if i try to remove either of them and put them in my inventory it prevents me from putting it back in the stash, my character just says "Impossible". The cubes still works though but i find it highly inconvenient.
Any idea how to potentially fix this?