I tried to rerender with different options you guys suggested, but nothing seems to work (except increasing render time 😄).
I think it's the material of the lamp or something.
When I move the lightsource down about 1 meter, I don't get the spots (but the light isn't coming from the lamp ofcourse).
If I change the color of the lamp housing to black, the spots also disappear.

Next, I tried to use a VRay material (plastic) but that didn't help neither.
It seems that whenever I change the lamp material to something bright, It shows me the spots. When I use a dark material, the spots disappear.

I'm at work now, so I can't show you the results right now.

Tnx 😄