To re-register Windows client/server in WSUS review the following instructions:
1. Run “gpupdate /force” command on the Windows client/server that have a registration issue in WSUS.
2. Run “wuauclt /detectnow ” command on the Windows client/server that have a registration issue in WSUS.
Tip: You can use the Event Viewer to review the re-registration
3. In rare cases, you may need to run: “wuauclt.exe /resetauthorization /detectnow” command on the Windows client/server that have a registration issue in WSUS.