I'm sure now that it is not a problem with the screen, it's the light sensor..
I was standing near a window and put my finger several times on the sensor and it clearly adjusted the screen brightness automatically.
Glad that it's not a problem with the screen, now we've only have to find a solution.
Oh, forgot to mention this :
I found a widget that you can add to your homescreen "On/Off backgroundlight". (not sure of the name though because my language is set to Dutch)
It's the first widget in the widgets list..
If you turn off the backgroundlight with this widget the screenbrightness is not adjusted automatically anymore
Downside is, the screenbrightness is then set as low as possible. But that's fine for the evening