If you take a look at the frontpage I have a list of categories, if I press on “Dekor” it goes to the right category, but if I then try to filter by category on the left it keeps the URL and adds another category which makes it not work.
It makes sense that no products are shown in this case. The product list is already filtered by wordpress (because of the /benders/ in the url). if you now use the product filter on the left and select for example granitt it narrows down the list even more to only show products that have both categories (benders and granitt) applied to them.
What can you do? Well… I wouldn’t show the category filter on the left at all when you’re already on a category archive page.