The consistent thread for both 16 versions is error code 0x80070643 for this particular problem.
Normally, when Windows Update (WU) goes wonky, recovery requires troubleshooting.
Wait a minute (60 seconds) before you do this or you’ll get an error message that says “You must wait for the previous update to complete …” But indeed it will proceed and complete successfully.
Over the past week to ten days, lots of users have been reporting issues with Windows Update and Windows Defender.
This applies to versions 1607 (Current Branch) and 1703 (Insider Preview aka Creators Update).
In 1703, clicking “Protection updates” produces this screen.
Next, please click the “Check for updates” button at the bottom.