To get to the point.
The ethernetmodule isnt working with the code from mysensors and I don't know why.
If I upload a webserver code for testing dhcp or static is working.
And ping wont make sense if the interface isnt working at all. ;-)
Are you sure you don't take out too much juice from the Arduino? With both radio and ethernet module attached... Could you try powering the eth-module separately?