If you are lucky and haven't already updated Garmin Express, DON'T DO IT!
Hey Garmin, when are you going to fix this?
I uploaded an activity successfully on May 31st from my Forerunner 910XT. Yesterday I updated Garmin Express and none of my activities between May 31st and today will upload. I wasted a good part of my day yesterday trying to figure out a way to make it work and have read alot of posts in this forum where very basic common sense advice is given that doesn't solve the problem.