Someone suggester earlier, but just got message the "Windows cannot find".

Someone else recommended running Reimage programme which found and repaired registry problems. Now seems OK.

WINDOWS\diagnostics\system\Apps" is not owned but specifies SDDL in component Microsoft-Windows-AppsDiagnostic, pA = amd64, nonSxS, PublicKeyToken = {l:8 b:31bf3856ad364e35}