WCF::handleException() Unhandled exception: Class 'wcf\util\HeaderUtil' not found

#0 /www/htdocs/v036211/forum/wcf/lib/system/session/ACPSessionFactory.class.php(35): wcf\system\session\SessionFactory->init()
#1 /www/htdocs/v036211/forum/wcf/lib/system/WCF.class.php(345): wcf\system\session\ACPSessionFactory->load()
#2 /www/htdocs/v036211/forum/wcf/lib/system/WCF.class.php(143): wcf\system\WCF->initSession()
#3 /www/htdocs/v036211/forum/wcf/global.php(25): wcf\system\WCF->__construct()
#4 /www/htdocs/v036211/cms/global.php(12): require_once('/www/htdocs/v03...')
#5 /www/htdocs/v036211/cms/index.php(8): require_once('/www/htdocs/v03...')
#6 {main}