[cflsystems]

there is no $product_links[num] in related_products.tpl (are you sure your version is 4.2.1?)
Here is related_products.tpl file

{*
$Id: related_products.tpl,v 1.20 2008/10/06 15:11:35 max Exp $
vim: set ts=2 sw=2 sts=2 et:
*}
{if $product_links ne ""}
  {capture name=dialog}
    <table cellspacing="0" summary="{$lng.lbl_related_products|escape}">
      {foreach from=$product_links item=p}
        <tr>
          <td class="item-number">#{$p.productid}</td>
          <td><a href="product.php?productid={$p.productid}"{if $config.Upselling_Products.upselling_new_window eq 'Y'} target="_blank"{/if}>{$p.product|amp}</a></td>
        </tr>
      {/foreach}
    </table>
  {/capture}
  {include file="customer/dialog.tpl" title=$lng.lbl_related_products content=$smarty.capture.dialog additional_class="uproducts"}
{/if}