Thanks Felix.
I added your suggestion to the .h file, actually just
#elif defined(__AVR_ATmega2560__)
#define RF69_IRQ_PIN 21
#define RF69_IRQ_NUM 0
since the #else part was already there.
I'm afraid it did not help. The sketch seems to run alright up until this line:
radio.send(TONODEID, sendbuffer, sendlength);
and then it stops. The receiving radio shows nothing received and it's impossible to try to send any more text unless the Mega2560 is reset. I've checked the receiving Node ID which TONODEID should contain and that's correct.