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Thank you very much for the reply, which holds the clue that was missing on the verification page:

mention of Hive in the website

Do I understand correctly that a cross-link between a Hive post and e.g. a YouTube upload would be sufficient as a proof that I'm the owner of both profiles?

Yes, that would be fine. You can just add the word "Hive". That would be enough.

Thanks for your assistance! I just added a link to my HIVE profile (and other platform profiles) to the links section on my YouTube profile: https://www.youtube.com/WoodyofmC
These are visible in the upper-right corner, on top of the header banner.

Again, THANKS a lot for your help and assistance — much appreciated!