Hi,
Thank you for clarification. I have found the problem. Because the results override was active, the non-post results are also passed to the theme. However it looks like, that there is an issue displaying them, when the plugin forcibly sets the post type of non-post type results to ‘post’.
Anyways, I have added this custom code to the child theme functions.php to bypass that:
Please keep this code there, and more it to the production as well. That should resolve the issue permanently.