Finally got past the error message by creating a new account for Garmin Connect. It just didn't work with my old account for some reason. Thank you for your help, Mario!
I am running into the same issue trying to setup Garmin Express a new iMac. I am able to find the device, and pair with it. When I am prompted to enter an account, I use my existing account credentials and then run into the server error.
I agree with THADANDERS, creating a new account isn't a feasible option as there is years of training data, routes, workouts, etc that I'm not willing to lose.
FWIW:
+ I'm using a FR60
+ I previously used ANT+ Agent on a Windows 7 PC.