I tested your code on my end and it works great. Could you please share the JSON or XML export of your field group so I can check your setup?
Also, you can always use the returned post as a post object like this:
<?php $post_object = get_field('kurs_zu_diesem_seminar'); ?>
<h3><a href="<?php echo get_permalink($post_object->ID); ?>"><?php echo $post_object->post_title; ?></a></h3>
I hope this helps. Thanks 🙂
