The Site Server was Blacklisted.
THIS IS ONE OF THE ENTRIES IN THE LOG FILE.
<event date='May 04 02:39:31' severity='2' hostName='ServerName' source='ConfigServer' module='aexnsagent.exe' process='aexnsagent.exe' pid='110736' thread='110760' tickCount='-863284062' >
<![CDATA[RequestPolicies failed: Unexpected response from URL http://ServerName.Domain/Altiris/NS/Agent/GetClientPolicies.aspx: The specified host resource has been blacklisted (-2147217405)]]>
Repeated resource merging during replication can cause active guid to be blacklisted.
This issue has been fixed with SMP 7.1
Workaround prior to SMP 7.1:
Delete from the AgentBlacklist table on both the Parent and Child servers and then delete the computer out of the 'All Computers' Folder and then update the configuration on the effected computer, which will get a new GUID and started working again.
To delete from the blacklist table run the following SQL query :
delete from agentblacklist where guid in (select [guid] from vcomputer where [name] = 'computer name')
Symantec Management Platform 7.0 SP4