I ran into a similar problem.  I found it to be some new xFi Advanced Security added by Comcast/Xfinity to your gateway/router.  You need to disable this service long enough to make the download.   You can access it thru your Account, but it wasn't easy to find.  Here's the path to take you where you can disable/enable.     https://internet.xfinity.com/more/my-services/my-services-disabling  Good luck. Herb WB8ASI