Error 0xC800021F

It looks like the client datastore failed to initialize.

1. Open a CMD prompt on the client.
2. Type "net stop wuauserv" (without quotes) .
3. Type "cd %Windir%\SoftwareDistribution".
4. Type "RD /s /q Datastore" (this will remove the client datastore).
5. Type "net start wuauserv" (without quotes) .
6. Type "wuauclt /detectnow" then check %Windir%\WindowsUpdate.log if it is successful.

OR, just stop the Automatic Updates Service and delete "%Windir%\SoftwareDistribution\DataStore" folder and start Automatic Updates Service and force the update detection (wuauclt /detectnow)