Verify your error-logging settings in php.ini or any file that sets it.
error_reporting = E_ALL & ~E_NOTICE & ~E_STRICT & ~E_DEPRECATED & ~E_WARNING
I suggest to really use only the plugins needed and disable object-statistics and session-statistics, too.
For real "Usage" number of concurrent users you should write your own script that queries ILIAS session table for activity in i.e. a time-window of 10 min. The session-statistic is depending on your "session expire" setting in client.ini.php and shows at least every user that had a session in the last 1800s (Default setting).