Ok ! I found the solution, this line was the cause of the problem :

<FlashFxLightCastsShadows value="false" />

it has been changed on true after installing OIV sound mod, so entirely my fault, THX for your answers !

@a63nt-5m1th thanks for these settings and explanations

@MegaDeveloper yep sad, but certainly the better way for a very good optimization

Hello guys !
Why shooting guns doesn't make light around like exhaust backfire or fire fx ? It's ugly

Is it possible to fix that ? thanks