I had the same problem, but just called MS Support, like they listed in the help section. I had to provide proof of purchase (which I luckily had moved from machine to machine since 2012). They sent me an e-mail with the log-in information.
I've heard back from emailing, but it's useless. They say, "take screen shots". Of WHAT? Of it saying, "it's failed"? There's no error code, nothing. As other posters in thread have said, we're a technical crowd (for me 30+ years in IT, and 15+ at FAANG) and we can't get this crap to work. I think it's WAI – they want us to repurchase.
(For the record, for me, it'll be like 10th time I've bought it, so it's working for MSFT. [The first time I bought it was 5$ to Notch's PP account.])
Take a screen recording of the page, with the devtools javascript console or network tab open. A technical crowd should know what's useful for troubleshooting :)
Everything was smooth.
Did you try that instead of e-mailing?