Bill Wright explained on 28/06/2018 :
at 12.07am approx BBC 4 HD (not SD) terrestrial (not satellite) stopped
transmitting 'The Fairytale castles of King Ludvig' and started transmitting
a card saying 'C Beebies Whoops we have a problem!'. I'm using Emley Moor.
Anyone else getting this? Is it just Emley or national?

I was watching it on SD via Emley Moor and that was fine. It was maybe
something to do with switching transmission over to the temporary EM
mast.