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: Fri, 22 Mar 2019 13:07:36 +0000
request: /index.php?form=UserLogin&s=9fbafbfc3de6189c827732a5a815d06497b1e51f
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/form/UserLoginForm.class.php(103): UserAuth::getInstance()
#5 /home/www/web55/html/hag/forum/wbb/wcf/lib/page/AbstractPage.class.php(77): UserLoginForm->assignVariables()
#6 /home/www/web55/html/hag/forum/wbb/wcf/lib/page/AbstractPage.class.php(36): AbstractPage->show()
#7 /home/www/web55/html/hag/forum/wbb/wcf/lib/acp/form/LoginForm.class.php(30): AbstractPage->__construct()
#8 /home/www/web55/html/hag/forum/wbb/wcf/lib/util/RequestHandler.class.php(63): LoginForm->__construct()
#9 /home/www/web55/html/hag/forum/wbb/wcf/lib/util/RequestHandler.class.php(94): RequestHandler->__construct('UserLogin', Array, 'form')
#10 /home/www/web55/html/hag/forum/wbb/index.php(9): RequestHandler::handle(Array)
#11 {main}