Ok, so, as I have two other Discourse sites, I tested that on another one, and it… worked !
Now there is something on my dessein-tech.com site with interferes with that mod.
I deactivated all components one by one to no avail. 
Ok, so, as I have two other Discourse sites, I tested that on another one, and it… worked !
Now there is something on my dessein-tech.com site with interferes with that mod.
I deactivated all components one by one to no avail.