Thanks Irina,
who on earth can tell where FP2’s list of ringtones comes from? 
It shows personal ringtones from my external sd mixed up with system-(?) tones that I couldn’t find anywhere on my rooted phone. Do you know where the system-rintones are stored?
For testing I copied a system-tone called “Ganymede.mp3” to storage/emulated/0/Ringtones and renamed it in “ganymedeIntern.mp3”. But it’s not even shown in the list. But that reminded me on the Ringdroid-app. So I opened Ganymede with Ringdroid and stored it as a ringtone named “Ganymede Rufzeichen”.
After I chose THAT in the Fritz!App Fon it is stored “reboot-proof” and doesen’t get lost again.
Cheers, Barney