i feel stupid now...YES! It works...

I'm sorry, no, it doesn't help.
This site is weird. I have made prototype on 1 server. Then copied it 1:1 to another server. There some links don't work. Previous page, Next page pagination and the Divi Theme builder only work when i set permalinks in Wordpress to "Simple". When i change permalinks to "Post Name" or any other, the site misbehaves. Can't figure it out. Maybe this is also the reason the site doesn't "listen" to my code or your code to expand the number of search results.
Thanks for the help but it seems the error is not in your plugin.

Does WPFTS limit number of search results?
I have set Settings>Read to 100 posts (in WP Admin)
i have added to my child theme's functions.php:
function change_wp_search_size($query) {
if ( $query->is_search ) // Make sure it is a search page
$query->query_vars['posts_per_page'] = 50; // Change 10 to the number of posts you would like to show
return $query; // Return our modified query variables
}
add_filter('pre_get_posts', 'change_wp_search_size'); // Hook our custom function onto the request filter

But i only get 5 results (always, for any keyword, even if there are more results)

Kind regards, Marco