Similar issues being seen here. I have Garmin Connect setup to send to Strava and Training Peaks, and that works fine. However, although the data is passing through Garmin Connect, the ride itself is not shown in activities in Garmin Connect. This is being observed after the 7.1 firmware upgrade to my Edge 520.
I cannot manually upload the .FIT file either. The progress indicator just spins. If I try to upload an old file then it says the file is already uploaded so something appears to be stuck somewhere.
Using Garmin Express I get "There was an error syncing with Garmin Connect". This is even though the activity went from the Edge 520 through to Training Peaks and Strava (via Garmin Connect).