Hello Mikko,
I'm investigating meter behaviour in unstable low speed GSM channel condition.
To describe in short - a lot of ping loss situation.
Just a few moments after every 'Data send failed. Try to resend message x/y ' I've got an unacceptable frame even in case of restored data exchange.
Still can't decide is it a meter firmware problem or a DLMS Director problem - some of them unable handle temporary tcp/ip link availability problem.