Well guys, no more problem after the Martin Hudy's tip
I'm using the gear diff one step tight and my rear belt was ok.
I'm sorry Salinet for being a pain in the a**, but I still don't understand what do you mean by "middle to tight"? Starting from the middle position (initial instruction manual position), how many steps left or right (tigher or looser) is that? Today my colleague also snapped his belt, his was in middle position, as is mine in the moment. Tomorrow is race day, and I will try with horizontal position (not upper or lower, the softest position) or the one tighter than that, since the belt has had a few runs (initial manual middle position) and it's already very worn on the side.
You got it.
You get the initial instruction manual position and tight one step only.