Related posts by taxonomies does not work , what could be the reason?

by

Your tax_query is using an invalid field type. Try:

$args = array(
  'post_type' => 'films',
  'post__not_in' => array($post->ID),
  'showposts'=> 3,
  'tax_query' => array(
        array(
            'taxonomy' => 'actors',
            'field' => 'term_id',
            'terms' => $tax_IDs
        )
    )
);

See the Taxonomy Parameters of WP_Query for more options if that doesn’t wok.