This could be hard to debug from afar but here are a few questions that might help narrowing down where the nodes are broken.
Does this also appear when doubleclicking an equipment item to unequip it?
Have you done any changes to SwapEquipItems?
Is SwapEquipItems running twice for any reason when you unequip an item?
Have you done any changes to the OnDrop function of BP_UI_InventorySlot?