When running rF2 with highest shadow settings i get a black rectangle below my car.
Lowering the shadow settings 1 step removes it.
Sometimes graphics drivers and some game settings don't go well with each other.
I don't remember from the top of my head but i assume that R3E has presets for graphic options?
Lower that to low or mid and see if the problem still exists.
If it's gone on lower settings, you could figure it out by changing settings step by step.
If this doesn't help, a clean uninstall / re-install of graphics drivers would be my next step
before re-installing the game.