Thanks for your solution. It definitely got me on the right track.
My working solution with ACF Post Object (Multi Selection) looks like this:
// Post Object variable
// In my case a subfield, because it's in a group
$acf_post_object = get_sub_field('acf_post_object');
// Get the ID of the Post Object
$featured_img_id = get_post_thumbnail_id( $acf_post_object->ID );
// Get the URL of the Post Object's featured image
$featured_img_url = wp_get_attachment_image_url($featured_img_id, 'thumbnail' );
// Use the url
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users