Hi Mike,
I had exactly the same issue last weekend. This affected all cameras. Everything working fine when left home, stopped at a motorway service station for 10 mins and would not function on start-up. Got to destination and still not working. Car was left for 4 hours or so and then worked fine again when started up to drive home. No issues since or before.
I had used the windscreen washers on the journey a short time before stopping at the service, but had not filled them up recently. Weather was dry.
Be interested to see what your diagnostic check reveals. Intend to run one myself.