Bingo! This is a backwards incompatibility - thanks for reporting!

@joaoabs You should be able to fix this by using the SecurityPersonalizer example from the 2.3.1 lib and copy/paste the settings section from your current sketch to the example sketch.
@Anticimex issue originates from replacing all signerSHA256xyz() functions in the cryptoHAL

@snyfir Can you run the same tests using the latest release, i.e. 2.3.1?

@m3nt4lll if the modules are 433Mhz you can use the pre-defined RFM95_434MHZ setting (center frequency is 433.92Mhz)

@m3nt4lll Are you using 868 or 915Mhz modules? How are the IRQ lines connected?

@berkseo Nice board and idea - would you mind posting your pcb layout files and testing sketch for the community?

@flopp yes, GW sketch + log, and if possible, from a node that stops sending messages

@flopp Please post the full debug log + sketch for further troubleshooting

@mfalkvidd Strictly speaking, only @Yveaux's solution would work without modifications to the MySensors core But the challenge was weakly defined, so no cheating in that sense

@alowhum Instead of a timer, the watchdog interrupt is used to set a flag on a (in this sketch) not-used eeprom addressing register (EEARL)...certainly not a generic approach, but functional

@Yveaux @mfalkvidd Now it gets ugly - and we are not talking ASM yet...

ISR (WDT_vect) {
  WDTCSR = _BV(WDCE) | _BV(WDE); 
  WDTCSR = _BV(WDIF) | _BV(WDIE) | 6; // 1s
  EEARL = 1;
} 

void setup() {
  WDT_vect();
}

void loop() {
    if(EEARL) {
      EEARL = 0;
      // ... code that runs every second...
    }
}

Flash: 502 bytes
Ram: 9 bytes