Errors found in the agent logs similar to this
"ERROR 0x4019 occurred while processing managed file C:\kworking\KMonitorsets\KMON$null.xml"
Delete the contents on this folder "C:\kworking\KMonitorsets" then perform a "Update Lists By Scan" ( Because it will reapply the monitoring) on the affected machine.
It's important that there isn't a large time gap between the deletion of the files and the execution of the "Update Lists By Scan". If there's a large time gap between the two the "KMON$null.xml" error will return. If you have this issue on a number of machines you can automate the process by creating an agent procedure. However it's recommended that you distribute the execution of this script over a large time period to prevent an overload condition on your Kserver.
EDIT Oct 2015
This maybe a more elegant way to resolve this issue