I can't login Express but got the message displayed "Invalid sign-in. Check that you have the correct Garmin Connect authentication domanin in your Express settings." The web site login normally but can't enter the dashboard.
Just in case anyone else has this issue, I managed to solve this problem.
Using a Mac, open the Keychain Access App, then lock and then re-open the default login keychain (click the padlock on the top left of the app). This immediately fixed the issue for me.
