You have to love the competent help you get through DJI Tech.

Might as well go ask at the local Starbucks shop if they can help you out.

Thanks for awsome service DJI!

Sorry to hear about that, Waldbjoern.
We'd like to help you double check it if it's convenient for you to confirm the model and specificationof that SD card.
As mentioned on our website, write speed ≥15MB/s, Class 10 or UHS-1 rating  are required for SD card on P4P and Class 10 or UHS-1 rating are required for SD card on Mavic.
For the case you reported, it may be caused by incompatibility problem since you've already tried the basic troubleshooting.
If it can not be recognized, then the best course of action would be to return it and change another one which works fine on aircrafts or you can use this card for other devices such as cameras or PC, then purchase a new one.
You can also refer to the supported SD cards list for P4P here (FAQ 2 on product section) : http://www.dji.com/phantom-4-pro/info#faq