WP_Query Cat parameter

Hi!

I am trying to sort destinations (post type destination) by category.

echo "Category ID: ".$category;

      $paged = get_query_var( 'paged' ) ? absint( get_query_var( 'paged' ) ) : 1;
        $args = array(
          'post_type' => 'destination',
          'posts_per_page' => $destinations_two_number,
          'order' => $orderpost,
          'orderby' => $orderby,
          'paged' => $paged,
          'cat' => $category
        );
        $wp_query = new WP_Query($args);
        while ($wp_query -> have_posts()) : $wp_query -> the_post();
        global $product;
        $params = array( 'width' => 555, 'height' => 370 );
        $image_thumbnail = bfi_thumb( wp_get_attachment_url(get_post_thumbnail_id()), $params );

Unfortunately I do not get any output. This http://visitadria.kreativkessel.com/destination/villa-lilli/ post should be definitly part of the cat 21!

Have I defined sth wrong in the code? Thank you for your help.

kind regards.