This can be the case when you migrate from old to new WSUS server.
Just run this on the clients and you are good to go.
REM BATCH FILE BEGIN
@echo off
Echo This batch file will do the following:
Echo 1. Stops the wuauserv service
Echo 2. Deletes the AccountDomainSid registry key (if it exists)
Echo 3. Deletes the PingID registry key (if it exists)
Echo 4. Deletes the SusClientId registry key (if it exists)
Echo 5. Restarts the wuauserv service
Echo 6. Resets the Authorization Cookie
Echo 6. More information on http://msmvps.com/AthifPause@echo on
net stop wuauserv
REG DELETE "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate" /v AccountDomainSid /f
REG DELETE "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate" /v PingID /f
REG DELETE "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate" /v SusClientId /f
net start wuauserv
wuauclt /resetauthorization /detectnow
Pause
REM BATCH FILE END