Janick

I am having the same error message

Thank you very much! We can see a significant speed improvement!

Thanks, the permalinks were not enable.

After some minor success programming myself I came across the code below.
It adds all custom content types to the loop.

https://bajada.net/2010/10/12/custom-post-types-in-the-loop-using-pre_get_posts-refined

function ucc_pre_get_posts_filter( $query ) {
  global $wp_query;

  if ( !is_preview() && !is_admin() && !is_singular() && !is_404() ) {
    if ($query->is_feed) {
    /* As always, handle your feed post types here. */
    } else {
      $my_post_type = get_query_var( 'post_type' );
      if ( empty( $my_post_type ) ) {
        $args = array(
          'public' => true ,
          '_builtin' => false
        );
        $output = 'names';
        $operator = 'and';

        /* Get all custom post types automatically. */
        $post_types = get_post_types( $args , $output , $operator );
        /* Or uncomment and edit to explicitly state which post types you want. */
        // $post_types = array( 'event' , 'location' );

        /* Add 'link' and/or 'page' to array() if you want these included:
         * array( 'post' , 'link' , 'page' ), etc.
         */
        $post_types = array_merge( $post_types , array( 'post' ) );
        $query->set( 'post_type' , $post_types );
      }
    }
  }

  return $query;
}
add_action( 'pre_get_posts' , 'ucc_pre_get_posts_filter' );