Error Message: Unable to load video

I've spent all day editing together some footage for a planned folio update over the weekend.

Testing out two different video sizes, I was disappointed to see the larger of the two wouldn't upload.
I tried again in another browser (Chrome, then Safari) - same result. I tried outputting the file with a slightly different codec, then tried other file formats, deleted the page and started all over again.

At the end of my tether, I searched 'Unable to load video' in the forum, only to discover many of us have been asking for a response about this issue for 18+ months, with literally nothing. How is this possible?

The two videos on https://murraygalbraith.com/test are generated by the exact same software just minutes apart, all the settings are exactly the same. Nice one, SQSP