Iam aware of that "bug or not bug " that you cant open stacked item , you need to move it to another slot from stacked so you can normaly open it ...
Its not perfect solution , that was only to handle exploit opening one chest more times ..
You can just put stack size 1 , and you will handle your problem ..
Iam still looking for the best way about this !
And yea , i dont know what XP plugin, thats from another plugin ...So not sure how that plugin works .
I make now update for stacked items , so you will not have that problem ...
You will be able to normaly open stacked chest(with safe triger for exploiting , opening same chest more times , getting xp for not opened chest) !
Iam testing it now with skill tree ..if all goes fine i update ! Not sure what plugin u use ?