WCF::handleException() Unhandled exception: Undefined constant "wcf\system\session\COOKIE_PREFIX"

#0 /homepages/31/d112644743/htdocs/schuco-forum/wcf/lib/system/session/ACPSessionFactory.class.php(27): wcf\system\session\SessionFactory->readSessionID()
#1 /homepages/31/d112644743/htdocs/schuco-forum/wcf/lib/system/WCF.class.php(345): wcf\system\session\ACPSessionFactory->load()
#2 /homepages/31/d112644743/htdocs/schuco-forum/wcf/lib/system/WCF.class.php(143): wcf\system\WCF->initSession()
#3 /homepages/31/d112644743/htdocs/schuco-forum/wcf/global.php(25): wcf\system\WCF->__construct()
#4 /homepages/31/d112644743/htdocs/schuco-forum/global.php(13): require_once('/homepages/31/d...')
#5 /homepages/31/d112644743/htdocs/schuco-forum/index2.php(9): require_once('/homepages/31/d...')
#6 {main}