H guys.
Occured a similar problem with me.
In NS show task server installed and in Altiris Agent of Site Server show plugin installed too, but when I checked task server "instances" in Site Server IIS, it not was created.
In IIS must have ths instances:
App Pools > DefaultAppPool > Altiris/ClientTaskServer
Web Sites > Default Web Site > Altiris > Client Task Server
Web Sites > Default Web Site > Altiris > Queue
This instances to me not appeared, so I uninstall the task service and install again.
I recommend you uninstall and install the task service manually in server.
Follow the link to help you with procedure.
http://www.symantec.com/business/support/index?page=content&id=HOWTO32603