Office 2019 account registration not possible (code 0x800704cf)?

I had closed a Microsoft account some time ago, but it was apparently still connected via my laptop. I have not been able to log into Office since then.

I bought a product key for Office 2016 2 years ago, probably via this email. Since new changes were no longer saved, I bought a product key for Office 2019 yesterday, apparently only for one-time activation. Office 16 thrown down, 19 put on again, activated. Registration still does not work. It is always said that there's no connection to the internet, although one definitely does. I've tried several things on the internet but nothing works.

After a long search, I found the simplest possible solution myself… ^^