Due with Ethernet shield 2, native usb set for serial, "dentro altro"

I have an Arduino Due with an Ethernet Shield 2. I use the Due programming port for downloading programs and the native usb port as a SerialUSB output.

I have a simple program that will write "Hello World" and the iteration number out to the SerialUSB port every second. When I have the network server running and connect to a web page, the output of the SerialUSB port continues to output the Hello World message every second, however I'll get about 100 or so lines of "dentro altro" output between them.

When I disconnect the the network connection by clicking off the web server page the USB output goes back to the one-second Hello world message.

I've tried turning on/off the Due ethernet control line (ETHER_CS, pin 10)for each message, and also running with the ethernet control on with the same results.

There are no instances of "dentro altro" in any of the code, so I'm guessing that it must be internal to the Arduino board.

Any ideas?