I get a Garmin Express sync error when trying to send a course to my Edge 25. Other syncing works fine as long as there is no course in the queue. If there is a course in the Garmin Connect queue, then Garmin Express gives a sync error every time until I delete the course from the queue.
My Edge 25 just updated itself to firmware 3.00, which may or may not have to do with this problem. (I did not attempt to send a course to the Edge 25 until after this update.)
Tech Support from the Garmin Express forum said this is a Garmin Connect problem and sent me here.