I have to admit a super stupidity which might be seen as a "beginner's stupidity". And that after 41 years of model flying.
I connect 2 Futaba R6303SB receivers and therefore I decided to leave it on "7 channel settings" in my transmitter.
Off course, and somebody told me on Facebook, I had to put it on "Multi channel".
Changed that and all is working fine now. I've just finished setting up my new PC-21...
Thanks for any help!