Hey, thanks for your support!
I found the issue. It was not about permissions (in fact, I also had that issue after I tried to tweak permissions, but that's more like I added troubles to troubles...). It was about not having correctly unzipped the different mods. I realized what I did after I compared with other mods that were working correctly. I did not put "what was in the /mods folder of the zipped file into the /mods folder of the game"; instead, I put what was zipped into the /mods folder of the game.
100% my mistake, thus.