Ok, I think I figured out why the filters were no longer displayed. Namely, I only assigned my products to their respective subcategories and not to the parent category as well. In this case the filters are not displayed.
As soon as I assign a product to a main category the attribute filters are shown. Is this normal? I think in many stores the products are only assigned to the subcategories, because they are automatically also displayed in the parent category in the archive. I guess it would make sense to adjust the filters so that they are displayed no matter at which level a product is categorized. Otherwise a lot of other users will run into this problem.