Fatal error: PHP warning in file /home/www/web55/html/hag/forum/wbb/wcf/lib/system/event/EventHandler.class.php (73): get_class() expects parameter 1 to be object, string given

Information:

error message: PHP warning in file /home/www/web55/html/hag/forum/wbb/wcf/lib/system/event/EventHandler.class.php (73): get_class() expects parameter 1 to be object, string given
error code: 0
file: /home/www/web55/html/hag/forum/wbb/wcf/lib/system/WCF.class.php (255)
php version: 5.3.29
wcf version: 1.0.10 (Horizon)
date: Tue, 26 Mar 2019 17:17:06 +0000
request: /index.php?page=Board&boardID=35&s=9f19d1c48d994c56d6ec7ab0c7b7731be824cdcf
referer:

Stacktrace:

#0 [internal function]: WCF::handleError(2, 'get_class() exp...', '/home/www/web55...', 73, Array)
#1 /home/www/web55/html/hag/forum/wbb/wcf/lib/system/event/EventHandler.class.php(73): get_class('UserAuth')
#2 /home/www/web55/html/hag/forum/wbb/wcf/lib/system/event/EventHandler.class.php(142): EventHandler::executeInheritedActions('UserAuth', 'loadInstance', 'UserAuth', 'loadInstance@Us...')
#3 /home/www/web55/html/hag/forum/wbb/wcf/lib/system/auth/UserAuth.class.php(28): EventHandler::fireAction('UserAuth', 'loadInstance')
#4 /home/www/web55/html/hag/forum/wbb/wcf/lib/system/session/CookieSessionFactory.class.php(34): UserAuth::getInstance()
#5 /home/www/web55/html/hag/forum/wbb/wcf/lib/system/session/SessionFactory.class.php(71): CookieSessionFactory->create()
#6 /home/www/web55/html/hag/forum/wbb/lib/system/WBBCore.class.php(201): SessionFactory->get()
#7 /home/www/web55/html/hag/forum/wbb/wcf/lib/system/WCF.class.php(91): WBBCore->initSession()
#8 /home/www/web55/html/hag/forum/wbb/global.php(18): WCF->__construct()
#9 /home/www/web55/html/hag/forum/wbb/index.php(8): require_once('/home/www/web55...')
#10 {main}