"Internal Server Error while processing this request" error on new WMI monitor

"Internal Server Error while processing this request" error on new WMI monitor

I've configured a new monitor to check service uptime via WMI but after updating/applying the threshold values I keep getting the error message "Internal Server Error while processing this request."
I can get a (correct) value returned when I test the monitor.
              New to ADManager Plus?

                New to ADSelfService Plus?