Thanks for your help, Raze, here is the situation :
Without HDR, the image is totally posterized and dark.
Please find a screenshot below.
[Linked Image]

And with HDR, I'm still having the same issue, even with deleting the graphicssettings file you gave me the path (thanks !)
When I enter the game, I've got the same issue as before :
[Linked Image]

When I go the HDR settings, that's a bit weird : the image gets way more dark, the settings are already Max bright/Min contrast , and when I cancel or accept, the image goes back to "normal" (like in previous screenshot). Here is a screen of the HDR settings :
[Linked Image]

Honestly, I'm close to giving up... already alot of time spent on this and still not working. That's a shame, I really wanted to play along with my kid frown