Login to https://connect.garmin.com/de-DE/ with Language "Deutsch" results in the Message "Disallowed Key Characters". Language "English" works fine. Tested with Mozialla Firefox 55.0b8 (32-Bit) and Microsoft Internet Explorer 11.
Login to https://connect.garmin.com/en-US/signin results in error message "Disallowed Key Characters". Can't get any browser or version to work. Very annoying. Tried Firefox 54.0.1 (Mac), Opera 12.16 (Mac), Vivaldi 1.10, Safari 10.1.2, Chrome 59.0 (Mac).
Same problem here: login to https://connect.garmin.com/en-US/signin results in error message "Disallowed Key Characters". Can't get any browser or version to work. Very annoying. Tried Firefox 54.0.1 (Mac), Opera 12.16 (Mac), Vivaldi 1.10, Safari 10.1.2, Chrome 59.0 (Mac). No more luck in iOS.
Wrote to Garmin Twitter. Waiting to see if I'll get a reply.
Finally found a workaround by connecting via Garmin Express and pasting the link in Firefox.