"Failed to export, if this happens again, please restart your app and camera"???

I've been using the Garmin Virb Ultra 30 and so far I've been super pleased with it until this evening.
When trying to export a 20 sec clip after editing, I get a "Failed to export, if the this happens again, restart the app and your camera"?
Well I've done that... Kept doing it and still no luck.
It will let me take a picture of the moment but its not quite as comical as the full 20 seconds (which really makes a difference).
The strange part is it was only this one video.... Other videos worked fine.
So I deleted the video hoping it was a just a bug but hey ho, it's decided to move to another video - same problem - same error message!
Could someone please give a more detailed answer as to WHY its failing to export rather than the app repeatingly telling me its failing to export.
I have a Hawuie P20... So its not exactly an old phone.
Many thanks