Same problem here! Garmin Express for Windows. When I want to add my Edge, it asks for a login, the form appears, I submit it, and then the rotating animation spins forever, making it completely unusable, because there's no way around logging in.
Note, that I tried submitting wrong credentials, which causes an error message. So errors work, success fails!
same problem - since garmin express update 6.9.1.0. i just updated to 6.10.0.0 - still the same problem.
i start garmin express -> Add a Device -> i can find my Forerunner 910xt -> Add Device -> Pairing -> Sign In -> "This may take a minute..." and thats it!!! it stuck at "This may take a minute..."
In my case, the device is found, the login to my existing account takes forever after submitting the credentials.. I guess the web login API is broken or became incompatible with Garmin Express..
Possible to try on a different machine and/or different account? Just wanted to narrow down whether it's your computer or your account or something else.