Followed suggestion here from blacktears (and John Baker in Q&A section) to use FlashPlayer export option (strictly speaking MPEG-4 > Flashplayer) and my test video has now uploaded correctly.
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IMO the note on the Upload Video - Step 1 page is misleading:
"Did you make your videos using "Vdlx" or "Video ProX"?
Then upload your video in "Flashplayer" export format for optimal quality."
I made my video using 'Movie Edit Pro MX'. OK, I'm guessing that 'Vdlx' means Videodeluxe.exe, which does apply to MEP MX, but it's still poor wording.
And is it right to assume a wider recommendation, namely that any MP4 video made in any Magix product should use this format? If not, what other 'rules' apply please?