The System event log could not be cleared and backed up due to the following error: Access is Denied.
This event is logged by EventSentry because an event log could not be cleared and backed up due to an access denied error. This is usually because the account the EventSentry service is running under does not have sufficient privileges. By default, the EventSentry services uses the LocalSystem which might have to be changed to a domain admin account.
The "Backup Event Logs" feature can be configured to schedule the backup and clearing of event logs.
You can configure this feature at "Packages -> System Health Packages -> Package Name -> Backup Event Logs -> Schedule".