Fatal error: PHP warning in file /www/htdocs/w0057807/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 /www/htdocs/w0057807/wbb/wcf/lib/system/event/EventHandler.class.php (73): get_class() expects parameter 1 to be object, string given
error code: 0
file: /www/htdocs/w0057807/wbb/wcf/lib/system/WCF.class.php (255)
php version: 5.5.38-nmm2
wcf version: 1.0.10 (Horizon)
date: Wed, 29 Mar 2017 07:16:01 +0000
request: /wbb/index.php?page=Thread&threadID=9034
referer:

Stacktrace:

#0 [internal function]: WCF::handleError(2, 'get_class() exp...', '/www/htdocs/w00...', 73, Array)
#1 /www/htdocs/w0057807/wbb/wcf/lib/system/event/EventHandler.class.php(73): get_class('UserAuth')
#2 /www/htdocs/w0057807/wbb/wcf/lib/system/event/EventHandler.class.php(142): EventHandler::executeInheritedActions('UserAuth', 'loadInstance', 'UserAuth', 'loadInstance@Us...')
#3 /www/htdocs/w0057807/wbb/wcf/lib/system/auth/UserAuth.class.php(28): EventHandler::fireAction('UserAuth', 'loadInstance')
#4 /www/htdocs/w0057807/wbb/wcf/lib/system/session/CookieSessionFactory.class.php(34): UserAuth::getInstance()
#5 /www/htdocs/w0057807/wbb/wcf/lib/system/session/SessionFactory.class.php(71): CookieSessionFactory->create()
#6 /www/htdocs/w0057807/wbb/lib/system/WBBCore.class.php(201): SessionFactory->get()
#7 /www/htdocs/w0057807/wbb/wcf/lib/system/WCF.class.php(91): WBBCore->initSession()
#8 /www/htdocs/w0057807/wbb/global.php(18): WCF->__construct()
#9 /www/htdocs/w0057807/wbb/index.php(7): require_once('/www/htdocs/w00...')
#10 {main}