to see that your dlclist.xml file is updated with your car dlc folder names? If you just recently updated your game version then you need to copy over the whole update.rpf file again and then update your dlclist.xml file and gameconfig.xml at minimum to get your addon cars working again.
A few weeks ago i "reupdated" my update.rpf folder, and it worked fine untill now. I have double checked that my car addons are in the right folders and my dlclist.xml i correct. I also tried to download a new version of gameconfig but no luck
I guess the only thing is to completely re-new my update.rpf once again.
It is just weird how it happened without me installing any new mods?