Problem solved!!
In fact, there is no problem here.
On this new generation of forklift, there is a "Seat belt sequence module". It's the little box with 3 green LED, near the VSM.
To start the engine, we have to sit on the seat first, then, to lock the seat belt, and only after we can start.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.