Here is a post that worked from one user on Answer HQ: https://answers.ea.com/t5/Technical-Issues-PC/Can-t-activate-my-mods-after-new-update/m-p/9560277#M190494
Repairing the game did not the trick. But I realized that the GameVersion file in my Documents folder were not the same as the real game version.
If you have the same problem, check in Documents>ElectronicArts>TheSims4 et find the GameVersion file. Open it, and see what number is written.
Open you game. Check at the bottom of the home page of your game. If the game versions are different, change the one in the file you opened just before for it to match the one written in your game.
