When I did a quick check around 4pm I couldn't get either camera image up on the TV. I noticed that the power lights on the "converter/multiplexer" boxes (little grey units above the black controller) were pretty dim.
I power cycled both grey boxes. When they turned back on the power lights were nice an bright and I could both view and control images from each camera in the usual way.
I'm not sure if this was the only problem with the right hall camera, or if Jack fixed that in the Hall earlier.
Anyway, both are working now.