Thanks for the reactions!
I’ve tried multiple denoisers. Unfortunately, it didn’t increase the quality of the image. It was a choice between a washed out, or an image with noise, fireflies and a bright area.
Also strange was that changing the settings (from YouTube) didn’t had a lot of effect. Also textures I had changed wheren’t visible (the charges) in the render.
After Cycles was set from GPU to CPU, changes became more visible. And after a while (I didn’t made a conscious change) the noise was almost gone. It becomes even more strange, because after changing the settings back a few steps didn’t bring back the noise.
I hope a Blender developer wil read this, because using such an unpredictable render engine for proffesional work, is very dificult.