Recently upgraded to 8.6 RU1
Error in Windows Application Logs
error invoking Dolnvoke in service http://<FQDN of NS>/Altiris/Workflow/ServerMonitoring.asmx
Release: 8.6 RU1
In this case, the Notification Server was in two Task Service Settings policies.
Settings > All Settings > Notification Server > Site Server Settings > Task Service > Settings
Removed the Notification Server from one of the two policies