Magento 1.8 - can't use backend after upgrading extensions

Question

I setup a fresh magento installation (1.8) and upgraded all extensions to their latest stable versions via Magento Connect Manager just moments after the installation. However, after it finished, I can't use the backend anymore.

It says: "Fatal error: Call to a member function setActive() on boolean in A:\wamp\www\magento\app\code\core\Mage\Adminhtml\Controller\Action.php on line 111"

Caching is disabled and I already cleared the folder "var/cache".

How can I use the backend again?

Thats the code from the error:

/**
 * Define active menu item in menu block
 *
 * @return Mage_Adminhtml_Controller_Action
 */
protected function _setActiveMenu($menuPath)
{
    $this->getLayout()->getBlock('menu')->setActive($menuPath);
    return $this;
}

Answer 1

Check your app/design/adminhtml/default/default/layout/main.xml file

...
<block type="adminhtml/page_menu" name="menu" as="menu"></block>
...

menu block definition should be there

If you think you have some extension overriding that try disabling them, one by one (so you can identify the responsible one), in app/etc/modules

Also, you can try downloading a fresh copy of your Magento version, and replacing all of your app/design/adminhtml/default folder