I think I do. I forgot to mention that if you use fulltext search, then this needs to be copied there as well.
Open up the search_content_fulltext.class.php file in the same directory as before, and go to lines 45-48, you should see exactly the same code as before:
} else {
$words = implode('|', $types);
$post_types = "($wpdb->posts.post_type REGEXP '$words')";
}
and replace it as before:
} else {
$words = implode('[[:>:]]|[[:<: ]]', $types);
$post_types = "($wpdb->posts.post_type REGEXP '[[:<: ]]".$words."[[:>:]]')";
}
I hope it finally solves the problem.