Try the following on the STB that you are trying to view recording on: DVR > Refresh Devices > OK. This should re-establish the connection.
If it does not, try an MRDVR Auto-fix from any box. Before running this option make sure that there are no recordings taking place or set to begin in the next several minutes.
DVR > Auto-fix MRDVR Issues > OK
On the next page select Fix/Activate My MR-DVR Service, press OK, press Right Arrow button.
On the next page press the Right Arrow button again.