Sometimes SMS_WSUS_SYNC_MANAGER may report error Message ID 6703:

WSUS Synchronization failed. Message: timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.

At the same time the rest of infrastructure is healthy. According to wsyncmgr.log an attempt to synchronize only lasts for 1-2 minutes. SUP reinstall does not help.

Go to WSUS manager\Options\Update Source and Proxy server. You may see the notification:

Cannot save the configuration because the server is still processing a previous configuration change.

This is a culprit! In order to get rid of it you may uninstall SUP, reinstall WSUS (including database) and install SUP. There is an easier way to fix it.

Start SQL Server Management Studio, connect to WSUS database and execute the query:

UPDATE tbSingletonData
SET ResetStateMachineNeeded = 0

Restart Windows Update service. Check to confirm that the warning has gone. The next scheduled synchronization should be successful.