Well that is weird. I don’t know if you’ve a lot of own materials in your library but just to try, maybe you can move them temporarily to another folder and try again. Not sure what location it is on the Mac but guess somewhere in documents as well.
I know there have been some issues if there were files with a .png extension for example but which were actually other format images.
If that doesn’t work you could also just rename the complete folder with all materials, just to have it to start again.
BTW, awesome car liveries! I love designing liveries as well for my online racing league hobby. It’s great what you can do combining some shapes that fit the car/team well.