FWIW - I have recently had trouble getting files from my 310xt and 910xt from the watch to Garmin Connect. There was uploading difficulties at the time I was connecting the watches resulting in two workouts being "stuck" on my watches (one workout on each watch - different workouts).
After much frustration and uninstalling and reinstalling Garmin Express to no avail I ended up downloading the old Garmin ANT+ Agent. While it wasn't seamless, I have managed to get the two "stuck" workouts from my watches to Garmin Connect and (more importantly) Strava.
I hope this may help the next frustrated Garmin user.