After a hard gear shift, a big boom sound was heard and I was unable to accelerate without having the engaged gear to fall.
It turned out that screw linking the rear bottom silent bloc to the engine was broken (please see the following image taken from the internet, in blue is the engine, in green is the chassis mount point, and in red is the broken screw)
The mount point in the engine (in red) is partially broken too.
Can this mount point (part of the engine) be changed by any mean without changing the whole engine?
How can the broken screw be removed from the remains of the mount point?
Please take a look at this video which show how to change the Silent Bloc. It is not the problem, but it shows where this part is exactly.