I've been having this problem for a few weeks now. Most of the time I can upload workouts via Bluetooth, but if I try to upload them wired from Garmin Connect, I just get the "Send to Device / Preparing Workout" window and the progress bar stays on zero. Someone else just posted the same issue on the 620 forum. Help ...
ETA - it won't upload via ANT+ to the 910xt either.