Cannot assign a new category or supplier to a product

Hi,

I'm currently working on upgrading our v3 site to 4.3.

So far I've done a clean install of 4.3. I've imported all the data and products using the store import addon. I've been in the process of migrating the skin/theme and fixing up my own addons to work with v4.



Everything seemed ok until today… I Tried assigning a category to an existing product. On the product page when I click on 'add categories' nothing happens (the popup is not opening).



Also, I cannot change the supplier. When I select 'supplier' the supplier box shows up, and says 'loading', the supplier list never loads.



It works ONLY when I create a new product. But once I save it I cannot change the supplier or categories.



Does anyone know what could be preventing these 2 things from working. Any help would be greatly appreciated.



Thanks!!