solution:
administrator/..../shop.product_details.php, line 86:
$q .= sprintf("AND FIND_IN_SET(#__{vm}_product.product_id, REPLACE(related_products, '|', ',' )) %s LIMIT 0, 4", (( CHECK_STOCK && PSHOP_SHOW_OUT_OF_STOCK_PRODUCTS != "1") ? 'AND `product_in_stock` > 0' : '')) ;
hope that helps!