cox contour2 unable to tune to channel, exit xre-00021 Error.

The last 2 nights at supper time I have been getting this error  unable to tune to channel-hit  exit xre-00021.  on 2 boxes but not the main box.  It fixes after rebooting, but why is this happening.  We just had are last issue worked on with the remote not responding which has been fine in the last week.