I can log in and the menus/sidebar work correctly. The activity history also does display as normal, although it is always empty (I use other software to view activities), so maybe someone else can comment on being able to view activities in Connect.
I guess it's best to contact the local Garmin Support (by phone, much quicker than email) and ask them to look into what the issue is with the account.
Thanks for the update. A bit puzzling though, when you register a display name, it is checked if it doesn't exist already, or at least that's what I thought.
This is often happening to me too.
Now on all browsers I try (Chrome, FF, IE, Edge). I tried deleting cookies, history, etc. with no success.
But some other day I can login without issues.
Will call the support.