Ok, found the problem. For anyone why is facing this problem, you need to make a extra change in skin1/customer/sendtofriend.tpl:
Change
Code:
{capture name=dialog}
<TABLE border="0">
<FORM action="product.php" method="post" name="send">
<INPUT type="hidden" name="mode" value="send">
<INPUT type="hidden" name="productid" value="{$product.productid}">
to
Code:
{capture name=dialog}
<TABLE border="0">
<FORM action="../product.php" method="post" name="send">
<INPUT type="hidden" name="mode" value="send">
<INPUT type="hidden" name="productid" value="{$product.productid}">