Problem solved!It turned out the unit was Atmega1268, instead of 644, as the original documentation said.
I had to dismantle it, because I have built it into the casing of a 16 port network switch, with the ethernet ports acting as input sockets. And - voila - the processor read Atmega1268....
It is now properly updated.
Thanks for the tip, Dmitri - and you're forgiven that the documentation sent with the unit said 644...