Starting a new game did not fix my old save. It's still dark. I also tried disabling shaders. It did not help with the old save but the flickering in new games has stopped.
Now, instead of the options "yes" and "no" the shader dropdown menu only shows "unsupported by hardware" if I am ingame. I can still choose between yes and no in the main menu.
I'm using a Geforce 560 TI, newest drivers.