It is quite famous error.
It means that somewhere you save object of SimpleXMLElement class (or its child) to the session. In your particular case it is Mage_Core_Model_Config_Element class.
When script ends his work, php tries to save all objects from $_SESSION array to the session file and tries to serialize $_SESSION array. Unfortunately, SimpleXMLElement can‘t be serialized because it wraps a libxml resource type. Resources cannot be serialized in php.
Probably, you are doing somewhere something like Mage::getSingleton(‘core/session‘)->setXXX(Mage::getConfig()->getNode(‘...‘)). Mage_Core_Model_Config::getNode() returns Mage_Core_Model_Config_Element, not just string.
So, you need to find this place and either add (string) type cast or use Mage::getStoreConfig(‘...‘).
综上，找到问题所在，在session里面保存了一个SimpleXMLElement配置相关的项，该项里面包含了一个libxml资源类型值，而PHP不允许资源序列化，因此抛出异常。
Magento遇到Serialization is not allowed
原文：http://my.oschina.net/lxrm/blog/412787