finally I was able to update.... I'm not sure what's wrong but worked for me... to be honest I'll try update normally tomorrow...
It worked also for me.
I'm under the impression that ANT Agent has some trouble uploading training sessions where the device has been used for a specific workout (Menu/Training/Workouts/Heart Rate/...). The upload button on Garmin Connect uses Communicator Plugin instead of ANT Agent and it works fine. Then, where there are no more workouts pending ANT Agent works again with "normal" activities.
I have reproduced and resolved the problem twice. But maybe it is just a coincidence.