In my wordpress installation I created a Flexible Content field called flexible_homepage_banners. This acf field has two layouts: banners_small and banner_large.
Banners_small is a repeater which looks for a custom post type called banner: banner_large also looks at this custom post type.
I’ve seem to fixed the issue by replacing the post object (custom post type) with a normal repeater.
I think it’s still possible to use post objects in combination with the flexible content field, but for now my problem has been solved.