Here is everything you need to know about why it happened and how the new fix works.
If your mods still don't show up in-game even after the key is accepted, the problem is likely the "Platform Launching" conflict with Steam or the EA App.
with fresh kits or gameplay overhauls, you’ve likely hit a brick wall: the dreaded "Encryption Key" prompt. This error pops up when the can't read the game's encrypted data, effectively locking you out of your mods.
: If the error persists after entering the key, go to Tools > Manage Mod Data in the manager and select Clear Mod Data . This forces the manager to re-read the game files using the new key. Troubleshooting Common Issues
FIFA 18 Frosty Tool Modmanager Tutorial | How to mod FIFA 18
: If mods don't appear in-game after launching, use the FrostyFix tool on GitHub . This tool forces the game to recognize modded data paths, which is often necessary for newer EA App or Steam versions.