DUE and Ethernet-Shield works only after a Reset?

Hello,
i use a DUE with a Ethernet-Shield R3. It works fine.
The DUE is powered over the USB-cable.

But after new connecting the USB-Cable on the programming port of the DUE, the Ethernet will not work. After a Reset of the DUE (open the serial monitor in the IDE or by pushing the reset-button on the Ethernet-shield) it will works fine until power off.

any ideas for a "power on working without reset" ?

Thanks, paulinchen