Seems that I was right, soldering the gateway and motion sensor nodes on a PCB solved the problem.
Best posts made by NickB149
RE: RFM69HW Raspberry Pi 2 Gateway
Thanks for your quick reply!
The solution was indeed to put the ./configure parameters on the same line; I also changed to a serial gateway which is now correctly recognized in Domoticz.
I did not need to change any pin definitions (yet).
Yeah, the two capacitors clearly shown are indeed not very effective for the radio module, there is however another capacitor directly soldered to the radio, which is a bit hidden in the picture.
Thanks again for your help!