I have generally seen this when CSPM was not querying the correct machine.
Sometimes CSPM Event Viewer will base it's queries on the machine selected, and sometimes
it will query the machines for which an alarm was selected.
So try this:
Stop and start Event Viewer.
open the Status Panel in the Event Viewer
(Be sure to not select any alarms yet)
Select the sensor, and select the menu option to view the Block List.
Also try selecting a specific alarm from that sensor and then select the menu option to view the Block List.
If you have a different machine selected in the Status Panel or you have alarms from different machines selected then CSPM will try to query those other machines and you would see that error if they are not running managed.