UPDATE; I spoke to Garmin's customer service department. Very nice guy, very efficient. And he told me they are aware of this issue, and it is a top priority with them. But he has no timeline for when it will be fixed, and based on their own troubleshooting message boards, it's been going on for at least three weeks.

He told me there is nothing he can do. He suggested that I try a different email (!) or a different browser. I tried two different emails, and both Chrome and Edge, and none of the combinations work.

What a mess! You'd think that they would tell people about this, rather than just continuing to sell these devices as if there is no problem.