i tried .. but it doesnt help:(..
i think the problem is at httpdocs\app\code\core\Mage\Core\Model\Layout.php on line 555 ---Call to a member function toHtml()
Hello
Magento probably wants to display a block that does not exist.
It may be something wrong in the layout files of your new theme.
To get an idea of what it might be put in the method Mage_Core_Model_Layout::getOutput() a debug line.
Put this: