The Task Service was removed from the affected server but the structure was left behind with incomplete references. In this case, it was the SMP itself showing the error.
The error can also be caused by deny rule is IIS " IP Address and Domain Restrictions" for specific IP or IPs.
Check if the following virtual directory exists: IIS Manager>Default Web Site>Altiris>ClientTaskServer OR IIS Manager>Symantec Agent>Altiris>ClientTaskServer
Check if you can actually browse to it and it opens: <install drive>\Program Files\Altiris\Altiris Agent\Client Task Server\ServerWeb
If the folder above exists but it is empty, re-install the Task Services on the SMP (or whatever site server that is showing the error): SMP Console>Settings>Notification Server>Site Server Settings, find the SMP on the Site Servers list and go to Services>Install/Remove services.
Check if there are " IP Address and Domain Restrictions" rules. If there are remove them. Reset of IIS is not needed.
Exaple of Deny rule for host 192.168.2.108
Deny rule itself, the one that has to be removed.
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