Error in articles

Hi,

We have the following error in all of our articles:

Fatal error: Mage_Core_Model_Session_Abstract::getMessages(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "Mage_Core_Model_Message_Collection" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/shop/public_html/app/code/core/Mage/Core/Model/Session/Abstract.php on line 215

Any idea?

Thanks

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