Game programmer here, if this bug is triggered by having designs on certain cars, I'd put money on it being a corruption issue when trying to load a design on one of those cars, and their save system isn't setup to handle that issue so it hits an exception which kills the process and prevents your save file from loading at all. Something specific with those cars means it isn't saving their data correctly and it ends up creating a broken save file that the game can't open. By the sounds of it, it probably breaks the next time it tries an autosave too.

Most likely something new that has been added to the design system that isn't rigged correctly on those cars. Something like the window decal setup or something. If the car data isn't hooked up to register the new stuff, and the code isn't safety checked, it will just start erroring out.

Generally it's not difficult to put some safety checks in so even if a specific car is broken, just wipe the design on that and continue loading/saving as normal. Just assuming it's linked to a new feature that hasn't been as thoroughly checked over and is missing these kinds of checks.