My guess is that it got something to do with the DNS and the name resolution. Maybe Garmin moved some servers. Then it should fix itself after a while when the records are updated. It can also be something…
I'm facing the same issue with Web on Firefox, so you're not the only one
This is the URL that gets a 401 Unauthorized. The Authorization header is being sent, so it is a problem on the server side
Can more or less confirm my theory. When I encounter this, multiple reloads of the page will not solve the issue. I just restart the browser to get another session, then the sleep page loads. It might be that different servers are on different versions of their API, incompatibility issue.