Can't add product to compare, _validateFormKey() return false

Question

I'm updating an old Magento instance from 1.7.0.2 to 1.9.2.1 (I know there is the v. 1.9.2.2).

I'm also using the Ultimo theme, but i tested the bug also with the standard RWD theme that comes with Magento 1.9.

The issue is about the add to compare button: it doesn't work. It only redirects me to the same page but it doesn't add the product to compare list.

I find out that the problem is in the addAction() function in CompareController.php

There is

public function addAction()
{
    if (!$this->_validateFormKey()) {
                $this->_redirectReferer();
                return;
            }
...
}

And $this->_validateFormKey() return FALSE so nothing is done.

Just for information, my add to compare link are looking like the following

http://example.com/catalog/product_compare/add/product/2480/uenc/aHR0cDovL3N0a25ldzE5Lmh0dGRldi5pdC9pcGhvbmUtNi1wbHVzLTE2Z2ItYmxhY2suaHRtbA,,/

At the moment, my solution is to comment the IF.

Somebody knows how to solve this problem?

EDIT:
Into the template file addto.phtml the code is the following:

...
<?php
    $_compareUrl = $this->helper('catalog/product_compare')->getAddUrl($_product);
?>
<?php if($_compareUrl) : ?>

    <li>
        <a class="link-compare feature feature-icon-hover first" href="<?php echo $_compareUrl ?>" title="<?php echo $this->__('Add to Compare') ?>">
            <span class="ic ic-compare ib icon-color-productview"></span>
            <span class="label"><?php echo $this->__('Add to Compare') ?></span>
        </a>
    </li>

<?php endif; ?>
...

Answer 1

You will need to change compare URL in your phtml

<?php
    $_compareUrl = $this->helper('catalog/product_compare')->getAddUrl($_product);
?>

This code will give you the right compare URL for your product.

If this will not work show your phtml code which have compare link.

Answer 2

I have an old overload in the local folder for the Mage_Catalog_Helper_Product_Compare Class, so it has not the new FORM_KEY feature in the function getAddUrl().

Thanks to @Akhilesh Patel