Found another clue.
I have a save_post action for saving the acf_fields in the normal editor (mainly for for Yoast and relevansii functionality). When I deactivate this function everything works fine. What I don’t understand is, that the function only acts, when !wp_is_post_revision($post_id) && !isset( $_GET[‘preview’] )
As far as I understand this function shouldn’t do anything while in preview mode.