Ok, strange thing
I have connected my setup today to check the commands for you and I2C seems to work fine. When I'm sending "M260 A3 B65" I can see the state changes according to address being sent:
Of course Duet is still saying "Error: M260: I2C transmission error", but now I know that is because I'm not sending ACK from my side.
So the last time I have probably connected something wrong, sorry for bothering you