Alright, problem solved!
If anybody else has the problem too:
1. I uninstalled all my other mods (made a backup)
2. Deleted my symbolic link
3. Made a new symbolic link
4. Reinstalled TruckersMP (after this it worked)
Don't know which one fixed it, but it worked after reinstalling TruckersMp and now it works as intended with my Singplayer mods