Updated March 11th, 2016
This is typically seen on Windows 2003 servers after they are rebooted when Host-Resources MIB is being used. After a reboot, some of the resources may be reenumerated and as a result, the OID being used by ipMonitor to monitor that particular resource is no longer valid.
The easiest way to avoid this is to download and install SNMP Informant Standard on the remote system and then use "SNMP (Informant)" as a communication type within the Monitor. More on SNMP Informant is available at http://www.snmp-informant.com/.