Just wanted to answer the question myself: I had to solder a decoupling-capacitor to the radios and everything worked: https://www.mysensors.org/build/connect_radio#connecting-a-decoupling-capacitor

Hi, not sure if @Floca get his nodes working, but for me, the problem got fixed with this #define MY_RF69_IRQ_NUM MY_RF69_IRQ_PIN as mentioned by @mfalkvidd Thanks!