I spent 3 or 4 hours day before yesterday chatting with several Norton experts without success.
But the solution I found myself is very simple:
Instead of running xfinity-master-installer_1.0.0.11.exe, which I got from
http://customer.xfinity.com/help-and-support/internet/downloading-the-norton-security-suite/
I simply used the older Norton_Download_Manager.exe. And it worked perfectly.