could it be that the failing agents running a lower LANDESK Servicepack version or even an older major release than your core? It sound like you try to update some elements in the agent that is incompatible. I would suggest to redeploy new agents to the affected clients.
Try to compare the following files:
Core: "C:\Program Files (x86)\LANDesk\ManagementSuite\ldlogon\ldiscn32.exe"
Client: "C:\Program Files (x86)\LANDesk\LDClient\LDISCN32.EXE"
If they don't match, you have to update your agents first.
The error you are getting is "The agent is not up to date".
The article above gives those return codes and what they mean.
I would suggest installing AV as part of the agent installation.
The document above may help you to troubleshoot this issue further.
Pay attention to the installation log files.
Otherwise, please contact support for a ticket.
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