Security Patch SUPEE-7405 Error

Question

After installing the security patch SUPEE-7405 I have got the following error in the backend: Mage registry key "_singleton/Mage_Core_Model_Domainpolicy" already exists

I tried to clear the cache, but no change.

Does someone know how to fix this?

EDIT: As a workaround I did the following:

   if(isset(self::$_registry["_singleton/Mage_Core_Model_Domainpolicy"])) {
        self::unregister("_singleton/Mage_Core_Model_Domainpolicy");
    }

in the Mage.php. This works for the moment, but does anyone know how to find out in which files the key is setted?