shows always only 10 post per page

Hello there,
also if there is selected to show 50 posts per page for my search results, there appear always only 10. (Also on the wordpress admin page for the post list)

I also tried to add this function to my child functions.php:
// ########## products per page in search
function myprefix_search_posts_per_page($query) {
if ( $query->is_search ) {
$query->set( ‘posts_per_page’, ’50’ );
}
return $query;
}
add_filter( ‘pre_get_posts’,’myprefix_search_posts_per_page’ );

I also tried to change the wordpress default settings for posts per page.
Unfortunately nothing worked. I tried to disactivate some plugins but also they not caused the problem. So i don’t know where to search anymore. Do you have an idea what could cause this, or how i could solve it?!?

Thanks in advance!!