WCF::handleException() Unhandled exception: Call to undefined function wcf\util\mb_strtoupper()

#0 /usr/share/nginx/html/forums/wcf/lib/system/cache/CacheHandler.class.php(116): wcf\util\StringUtil::firstCharToUpperCase('UserGroupOption')
#1 /usr/share/nginx/html/forums/wcf/lib/system/cache/CacheHandler.class.php(77): wcf\system\cache\CacheHandler->getCacheName(Object(wcf\system\cache\builder\UserGroupOptionCacheBuilder), Array)
#2 /usr/share/nginx/html/forums/wcf/lib/system/cache/builder/AbstractCacheBuilder.class.php(38): wcf\system\cache\CacheHandler->get(Object(wcf\system\cache\builder\UserGroupOptionCacheBuilder), Array)
#3 /usr/share/nginx/html/forums/wcf/lib/system/session/SessionHandler.class.php(156): wcf\system\cache\builder\AbstractCacheBuilder->getData(Array, 'usersOnlyOption...')
#4 /usr/share/nginx/html/forums/wcf/lib/system/SingletonFactory.class.php(27): wcf\system\session\SessionHandler->init()
#5 /usr/share/nginx/html/forums/wcf/lib/system/SingletonFactory.class.php(56): wcf\system\SingletonFactory->__construct()
#6 /usr/share/nginx/html/forums/wcf/lib/system/session/ACPSessionFactory.class.php(28): wcf\system\SingletonFactory::getInstance()
#7 /usr/share/nginx/html/forums/wcf/lib/system/WCF.class.php(345): wcf\system\session\ACPSessionFactory->load()
#8 /usr/share/nginx/html/forums/wcf/lib/system/WCF.class.php(143): wcf\system\WCF->initSession()
#9 /usr/share/nginx/html/forums/wcf/global.php(25): wcf\system\WCF->__construct()
#10 /usr/share/nginx/html/forums/global.php(13): require_once('/usr/share/ngin...')
#11 /usr/share/nginx/html/forums/index.php(9): require_once('/usr/share/ngin...')
#12 {main}