Fix for http://christianvarga.com/how-to-get-submenu-items-from-a-wordpress-menu-based-on-parent-or-sibling/#comment-2709818300

my_wp_nav_menu_objects_sub_menu.php

<?php

// add hook
add_filter( 'wp_nav_menu_objects', 'my_wp_nav_menu_objects_sub_menu', 10, 2 );
// filter_hook function to react on sub_menu flag
function my_wp_nav_menu_objects_sub_menu( $sorted_menu_items, $args ) {
  if ( isset( $args->sub_menu ) ) {
    $current_id = 0;
    $root_id = 0;
    
    // find the current menu item
    foreach ( $sorted_menu_items as $menu_item ) {
      if ( $menu_item->current ) {
        // store the current menu item
        $current_id = $menu_item->ID;
        // set the root id based on whether the current menu item has a parent or not
        $root_id = ( $menu_item->menu_item_parent ) ? $menu_item->menu_item_parent : $menu_item->ID;
        break;
      }
    }

    // find the current menu items children
    foreach ( $sorted_menu_items as $menu_item ) {
      if ( $menu_item->menu_item_parent == $current_id ) {
        // it has children, make sure the root id is current menu item
        $root_id = $current_id;
        break;
      }
    }
    
    $menu_item_parents = array();
    foreach ( $sorted_menu_items as $key => $item ) {
      // init menu_item_parents
      if ( $item->ID == $root_id ) $menu_item_parents[] = $item->ID;
      if ( in_array( $item->menu_item_parent, $menu_item_parents ) ) {
        // part of sub-tree: keep!
        $menu_item_parents[] = $item->ID;
      } else if ( ! ( isset( $args->show_parent ) && in_array( $item->ID, $menu_item_parents ) ) ) {
        // not part of sub-tree: away with it!
        unset( $sorted_menu_items[$key] );
      }
    }
    
    return $sorted_menu_items;
  } else {
    return $sorted_menu_items;
  }
}

Great!
Thanks!