I updated from 188.8.131.52 to 1.9.2.
After that first a SQL error showed up, admin panel was still working. So I decided to do: renew index management and clear cache.
Then it was working again, a few minutes later problem came.
Fatal error: Class 'Mage_Core_Model_App' not found in /var/www/clients/client1/web1/web/app/Mage.php on line 670
Anyone know how to solve this?