This issue occurs when OfficeScan agents with different IP addresses report to the server using the same MAC address. The server fails to identify the agent as a new agent, and prevents them from registering appropriately with the server because that MAC address is already in use.

To resolve the issue:

1. Make sure you have installed OfficeScan 11.0 Patch 1 or later.
2. Open the ..\Trend Micro\OfficeScan\PCCSRV\Private\ofcserver.ini file.
3. Look for [INI_SERVER_SECTION] and add the following line:

   [INI_SERVER_SECTION]
   EnableCheckClientMacAddress=0

   Where:

   0 = Allows OfficeScan agents to report to server using the same MAC address
   1 = (Default) Prevents OfficeScan agents from reporting to server using the same MAC address

4. Save the file.
5. Restart the OfficeScan Master Service.
6. Perform a manual update on one of the affected machines.
   Any machines with the same MAC address should now appear on the web console.