Error XRE-03121 on XiD-P

Started getting error code XRE-0321 on a XiD-P cable box several weeks ago.  I have three cable boxes (each a different model), and this problem occurs only on this one cable box.  It effects all channels.  However, when I hit the Xfinity button and go to Apps, I can still receive Netflix and other streaming apps.  Researching this problem through Xfinity Assistant suggested two solutions that worked once:

1.  Do a System Refresh

2.  Unplug the cable box for 2 minutes and plug it back in.

Each of these solutions worked once.  Then after a couple of days the error returned.  Once, the problem cleared by itself, but the problem returned after a day or so.  So, I went to a Xfinity retail store and swapped out the box for a new XiD-P box.  The new box has the identical problem. 

I have called Xfinity support several times about this error code.  A couple of times the agent was able to clear the problem, but as usual, in a couple of days the same error returned.  The agents were unclear about exactly what they did to clear the error.

I have seen this problem reported on multiple Forum complaints but have found no suggested solution that actually works.