I'm unable to view any activities (mine or others) on the main Garmin Connect website. Looking at debug in my browser, I see a 404 error on the resource /modern/main/js/marine/js/properties/marine_main_en.properties which sounds like it may be important.
It was broken in everything except for Edge on Windows. Tested Safari, Chrome, Firefox, IE on macOS and Windows 10 (and Safari in iOS for good measure).
But it looks like just now (like 1 minute ago) it started working again in all browsers. Not sure what changed, the marine_main_en.properties resource is still 404.