Thanks for your reply!
Yes, I have removed the tail light and also the plastic plug, under the tail light, that hold the bumper.
I have also done one more try, found one more screw, but it is still stuck. It seems like there is a black plastic thingy that holds it at the same place as before. See the pics.
Blue = Side of bumper
Yellow = Car
White: Black plastic thingy
Near the back wheel
Near the back of car
Any ideas? Should the black thing come of the car or should the bumper come of the black thing?