Even when i had a custom game config installed it allowed me more cars but after the 20th one i got this error then i did what i said and i have 50+ car mods installed and working!

I got an even easier solution i started getting this error too here is how to fix go to the dlclist.xml file inside there you will find this <Item>dlcpacks:\mpimportexport</Item> delete it then your addon car mods will work fine but it will mean sacrificing the import/export dlc cars but this method will stop the error