Found the answer: I went into the Master Repository (Menu - Software - Master Repository), then clicked 'Pull Now' at the bottom of the page and received a different error:
Failed to load SiteStat.xml
Which led me to the following thread:
The original poster in that thread states in the 20th post that he found the problem was his iPrism webfilter. We switched to an iPrism two months ago, and it hadn't required any configuration changes to allow updates. Until last week apparently. The iPrism includes a subscription to a black/white lists which rates websites as acceptable/objectionable, and to a lesser extent, safe/risky. I can only assume that their rating of the McAfee update site changed last week.
The solution was to add an exception for our ePo server.
That's good information - thanks for posting it