I'm having the same issue, and have been trying to find a solution for about a day now, If I find anything I'll let you know but after looking around, a couple of tips that might work for you:
1. Make sure your dlclist.xml has this line: <Item>dlcpacks:/MODDING/</Item>?
2. Download the latest gameconfig file, https://www.gta5-mods.com/misc/gta-5-gameconfig-300-cars
Dunno if these will work but anything is worth a try.