When I operate the switch of the light in my attic (fluorescent lamps) everything works perfectly, but when I turn off the light, the RCD switches off. How is that possible?

Answer

It is not easy to answer this with certainty because there are different types of fluorescent fixtures, but it is probably an induction phenomenon.

You probably have older fluorescent fixtures that contain a large inductance (choke coil). That coil is needed during lamp start-up, but also limits the current when the lamp is on. During switching off you suddenly interrupt the current, so that an induced voltage is created in the coil. Presumably this induces a small current in one of the nearby wires (possibly from another device).

Your RCD is constantly comparing the incoming and outgoing current. If there are no leaks, the incoming current is equal to the outgoing. As soon as there is a leak to ground somewhere (for example, if you touch a live device), there is more incoming current than outgoing and the RCD shuts off the current. That imbalance between incoming and outgoing current can also be caused by the induced current when switching off your fluorescent lamps. The RCD misinterprets this as a leakage current.

How to solve?

You can place a capacitor across the switch, so that the current is interrupted less brutally.
http://www.emcuk.co.uk/awareness/Pages/MitigationTechniques/Design/SuppressArcSpark.htm

Or you can switch to modern luminaires with an electronic ballast.