Today I found the solution for my problem. When the nitrokey is connected to the Motorola Moto G3, the voltage is pulsing all few seconds. This might happen, if the phone has a protection circuit that is activated by the power need of the nitrokey. So maby the Nitrokey needs a higher amperage at the beginning of the connection.
Me (and my dad, who helped me with it) were able to rebuild the pulsing problem with a 50 microfarads capacitor (as a replacement for the Nitrokey). Because a capacitor has a big need of amperage in charging phase, this is what activates the protection circuit. Maby the Nitrokey has also such a capacity.
To limit the current we placed a resistance of 10 Ohm in the OTG Cable, and suddenly the Nitrokey is detected by the phone without any problem.
So if the Nitrokey is not detected by a OTG Device, this could be a problem, and it could be (provisionally) fixed by adding a resistance.
The OTG Cable is not USB conform anymore with this modification. The secondary voltage went down on 4V, but as long as I tested, everything works.
We tried 22 Ohms first, wich leades to a 3,1V secondary and wich was too less to let the storage of the Nitrokey storage work. However, the smartcard and the keyboard were still detected.
If you have any questions on this, don’t mind to ask me. I’ll answer with pleasure.