I have a workaround that works for me (macOS, HighSierra):
- Go to the Garmin tab in activity mode. --> The screen is white.
- Press the small triangle next to the tab title ("Garmin")
- Go to the URL input field and press Return. --> The Garmin page appears.
- Press the login button in the Garmin page --> the screen is white again.
- Press Ctrl and click on the white screen (right mouse click). A popup windows appears with several options.
- Select the back option. --> The Garmin Login page appears and you can log in.
Christian