Occasionally, SonicWall firewalls will block WSUS downloads.
You can correct this by accessing a "hidden" page in the SonicWall interface,
as follows:

1.  Logon to the SonicWall interface.
2.  Change the file name at the end of the URL to diag.html.
3.  Check the "Enable HTTP Byte-Range requests with Gateway AV" checkbox.
4.  Click the Apply button to save changes.