Magento ‘Exception’ with message ‘Unsupported data type N’ in lib/Unserialize/Reader/ArrValue.php:88

After SUPEE-8788 patch Magento shows Cannot add the item to shopping cart. error message:   The following is logged into exception.log: exception ‘Exception’ with message ‘Unsupported data type N’ in includes/src/Unserialize_Reader_ArrValue.php:88 Stack trace: #0 includes/src/Unserialize_Reader_Arr.php(102): Unserialize_Reader_ArrValue->read(‘N’, ‘;’) #1 includes/src/Unserialize_Parser.php(53): Unserialize_Reader_Arr->read(‘N’, ‘;’) #2 includes/src/Mage_Core_Helper_UnserializeArray.php(44): Unserialize_Parser->unserialize(‘a:7:{s:4:”type”…’) #3 includes/src/__checkout.php(27996): Mage_Core_Helper_UnserializeArray->unserialize(‘a:7:{s:4:”type”…’) #4 includes/src/__checkout.php(28170): Mage_Rule_Model_Abstract->getConditions() #5 includes/src/__checkout.php(31075): Mage_Rule_Model_Abstract->validate(Object(Mage_Sales_Model_Quote_Address)) #6 includes/src/__checkout.php(31101): Mage_SalesRule_Model_Validator->_canProcessRule(Object(Mage_SalesRule_Model_Rule), Object(Mage_Sales_Model_Quote_Address)) #7 includes/src/Mage_SalesRule_Model_Quote_Freeshipping.php(74): Mage_SalesRule_Model_Validator->processFreeShipping(Object(Mage_Sales_Model_Quote_Item)) #8 includes/src/__checkout.php(35085): Mage_SalesRule_Model_Quote_Freeshipping->collect(Object(Mage_Sales_Model_Quote_Address)) #9 includes/src/__checkout.php(33436): Mage_Sales_Model_Quote_Address->collectTotals() #10 includes/src/__default.php(10589): Mage_Sales_Model_Quote->collectTotals() #11 app/code/core/Mage/Checkout/controllers/CartController.php(223): Mage_Checkout_Model_Cart->save() #12 includes/src/__default.php(14333): Mage_Checkout_CartController->addAction()… Read the rest