I updated a Magento shop from 18.104.22.168 to 22.214.171.124 version and now I can't add products to cart and I receive warning every time I reload product page. It's in the function
_generateAction($node, $parent) in
Mage\Core\Model\Layout.php and when I add log I get for helper checkout
Mage_Core_Model_Layout_Element Object ( [@attributes] => Array ( [helper] => checkout/ ) )
I am confused, don't know really how to look for solution and where to start from.