I tested a relation to temperature changes by disconnecting the hotend fan while the z probe was triggering at 220 degrees. But the z probe stopped to trigger immediately.
After I let it cool down again I saw a little "nugget" of dust at the outer side of the fan which was not standing out of the fan before. When I pulled it out some fine fibres (thin like a hair) of plastic followed the dust nugget. It looked like the fan had sucked in some plastic fibres from stringy prints and they combined with normal dust to build up this nugget.
After removing the nugget I had an idea and tested the z probe again. It was working fine again.
My theory is that the dust nugget slid between the fan blades and the housing while I removed the smart effector from the printer. Afterwards it slowed down the fan and somehow, since it had to work harder, it maybe heated up internally, leading to stronger magnetic fields emitted (I have no idea if that is nonsense or possible) which influenced the z probe. This would be an explanation why the effect was slowly building up but immediately stopping when the fan was stopped.
So my problem is solved now and I hope this thread will be helpful to others running into this issue.
Thank you for your answers. 🙂
