Oddly, since posting this message, I charged the car to full overnight and I haven't seen that message again! Even driving the car so that the battery went from full to empty. Still no message.
However, since then I have seen the MMI rebooting regularly and the SoS warning message once (also the navigation system is stuck on the initialisation screen permanently).
I've seen online someone reporting that the MMI rebooting can be prevented by logging on as guest rather than a logged in user. I've yet to try this myself so don't know if this works. The downside is you lose any phone app functionality you rely on (preheating, etc.)