Since posting the above a week ago, I have received no response from anyone at Comcast, even though "ComcastJoe" specifically asked to be advised of anyone still having this problem.
At the moment, using the same computer/internet connection referenced above, the following streams all produce Error 900|101000.101002.101005 messages:
http://xfinitytv.comcast.net/live/network/msnbc?cmpid=livetv_msnbc_onnow
http://xfinitytv.comcast.net/live/network/foxsports1?cmpid=livetv_fox-sports-1_onnow
http://xfinitytv.comcast.net/live/network/pac12network?cmpid=livetv_pac-12
Attempting to view the same streams on the official Xfinity Android app on a Nexus 7 tablet wirelessly connected to the same internet connection also does not work; it appears to quickly got through the authentication process, then hangs forever at "Preparing for playback . . ."
As before, the ESPN channels work.