Encountered this problem recently when trying to monitor a read only DC in my SCOM console. The server kept showing as Not Monitored.
Upon looking in the Operations Manager event log I had EVENT ID 21023 constantly appearing. Everything else in my environment looked ok.
Tried restarting the System Center Management Service to no avail, also tried stopping the service, deleting the Health Service Folder and starting it again. This didn’t work either 😦
Eventually the following fixed the issue:-
1. On my RMS I stopped the System Center Managment Service
2. Deleted the Health Service State Folder
3. Started the System Center Management Service
Then I restarted the System Center Management Service on the Management server that my monitored server (Read Only DC) was reporting to.
Lastly I restarted the System Center Management Service on my monitored server (Read Only DC) and the problem was resolved!