HIk NVR and Multi-Screen mode only accepting substream

Hi all. 2wk old NVR7608 just did something 'new' today by giving message bottom right of browser screen of "Multi-screen mode only accepts live view and playback at sub-stream. For any other stream type, please switch to single-screen mode."
I understand what is means but why has this just started appearing (no change to firmware, etc) .. and its making me think I have accidentally changed a setting?

I can go to single screen view and get main-stream or 4x screen and see 4 cameras on...

HIk NVR and Multi-Screen mode only accepting substream