make sure all cars and tracks is the selected header on the car/track selection screen
I did. I also resubscribed to all the official content available and checked if there are options under the "content" tab ingame. Also tried to subscribe to a couple of cars only, without success.
I finally reinstalled the game and after the initial bootup and typing in my name, the game now just closes immediatly after startup -> refund
New UI seems to cause crashes for some people, at least according to steam and the official forum.