This issue is fixed in Sportablet 9.0 / Uploader for Garmin 8.0, released today (July 23 2016).
Still: [today] Garmin Connect seems to be having problems logging in people. I've manually confirmed this by logging in and out their website repeatedly using Chrome. Overall, at least 2 attempt every 5 fails, sometimes 4/5. I suppose they're doing some weekend maintenance as they often do.
Bottom line: if you're having problems in the first phase of sharing the activity, when my app is displaying "signing in", retry. I just had to retry 4 times before it logged me in. After that, everything went smooth. Hopefully, they will solve the login problem soon enough.