I think I've narrowed it down to encryption, disabling it on the GW and the feather seems to make everything work ok. I've double checked that the key is identical but still won't work.
Encryption works when I use the ESP32 as "client" instead of the Feather M0. I thought the AES encryption was done in the radio (rfm69) and not the microcontroller (ie. ESP32/Arduino etc.).
Also, there's really no log message that indicates wrong encryption key or anything related to an encryption problem in the communication? :|
