I had the issue once, on the first proper journey! It said it was OK to continue, but take it into service ASAP. A small orange warning symbol came on in the bottom left of the screen for the rest of the 75-mile trip. Next start-up, it was gone.
I rang the salesman, and he said new cars often come up with this, and it usually goes away.
...But, on a 300-mile trip to the Lake District, Rex-ing all the way, it came on again. And stayed on overnight, but then cleared. I took it into Lloyds BMW in Carlisle, and they took very
detailed notes of its comings and goings to send back to BMW. (They also showed me the records of every 6 minutes' motoring
I'm going to take it to the agents I bought it from ASAP to see if they can clear the fault.
(Had the car a week, and already got 900+ miles on the clock! It's even better than I dared hope
. A full report on Rex ranges elsewhere.)
Noteworthy cars I've owned previously: Lancia Aprilia ... 20kWh i3 Rex.