miketsukamibo: There was indeed an error in the url. The message was that my "previous email" was already taken, which makes no sense. My account settings were showing my "current email", though.
That's right. I think when we create our accounts at first, a zendesk support account is created as well as the store one. Then for whatever reason some people who change their email later on end up with a mismatched login because the zendesk login wasn't updated properly. Other people seemingly haven't had issues changing their email address, so it's not likely a very widespread problem.
Changing your store email back to your original may indeed work to bring both logins into alignment, but I don't know if you'll have a problem if you change it again in the future. In my case the old email ended up on a spam/scam list, so I had to change it and keep it changed to a new one. Fortunately support righted my support login quite quickly.
miketsukamibo: Anyhow! Thanks for pointing me towards the url!
You're both very welcome.