Yeah, i got it !

I had to uncheck the "use displacement map" in the render settings to make it magically disappear. Anyway, much thanks for your support, if one of you has time to explain to me the why and how, i'm still curious.

 

edit: i changed the displacement setting in the material room for head and mouth skin to 0, so i can still use displacement maps in the same scene. What a shame, I lost hours on this thing, haha!