Found the Bug, one of my Inventory Utility Functions reacts badly to moving the same Slot to multiple target Slots, which can happen if not the whole stack is moved at once.
This Version should probably be working for you. ^^
(note 1: The scary sounding Description is only there to make sure people don’t use that Version lightly.)
(note 2: if you use this Version more often, make sure your Browser didn’t cache the Download)