https://www.windows-noob.com/forums/topic/7522-wsus-sync-sccm-error/

    deselecting all classifications in SUP (SCCM: Administration/Site/Primary Site/ (Configure Site Components/Softwar Update Point/Classifications/))
    Manual Sync in SUP



1) unselect all Classifications in SUP
2) Perform wsusutil reset
3) Perform sync
4) Re add the Classifications.



"wsusutil reset" in WSUS (C:\Program Files\Update Services\Tools)
    selecting classification in SUP
    Manual Sync in SUP

log: c:\Program Files\Microsoft Configuration Manager\LOgs\wsyncmgr.log