There is one rule in the category checks the a.logs for severity 1 errors. The XML parser errors was due to invalid character existing in the a.logs.
You can choose to disable the rule.
In the Altiris Console, go to Tasks> Monitoring> Monitor Solution> Manage Monitor Packs> Notification Server 6.0 - Basic> NS Log File> "Warning Alert Severity 1 Error"
On the Notification Server, update the Altiris Agent's configuration.
Investigate the cause of the invalid character in the a.logs.
Please refer to