Quite strange indeed.
As requested I erased the file and it runed smoothly until the file of Augsut 8th, 2015.
I, then erased also this file from the device and was able to sync up to August 13th, 2015 training.
Then one more time blocked, that time on August 17th, 2015 training (One more time a nearly 3 hours training).
This trainings erased from the watch one after the other with a sync between each erasement, everything is "back to normal" ......until when?
My fisrt thought was that the issue was due to a duration matter (around 3 hours training) but I also noticed that it is always on road cycling training.
Never on other recorded kind of training (Gym, hiking...)
I will keep you posted if the problem occurs again.