This is caused by something you have done in your project.
This is a bug with Unreal starting in 4.11 and I hope is fixed in 4.14. When the UMG Widget is hidden (after looting everything) it is no longer resetting the IsHovered UI states which is causing issues with the IsMouseOverUI Logic.
This is an Engine UMG Bug and not a bug with ARPGIS but I have provided a work around for now until Epic fixes it by basically disabling the IsMouseOverUI logic in the InventoryPlayerController.