This is a wild shot, but is your additional GUI OVER the inventory window? Then it will "catch" the mouse clicks and the result will be "inventory clicks not working".
I'm asking because your code looks nothing out of the ordinary and shouldn't break anything, and I HAVE seen GUI overlays causing the exact problem.
I'm asking because your code looks nothing out of the ordinary and shouldn't break anything, and I HAVE seen GUI overlays causing the exact problem.