Creator won't start when USB is connected

I just bought a new main board for my Creator 1 and installed it. Everything works - as long as I do not have the USB cable connected when I turn the printer on. If I turn on the printer and then connect to the USB port, everything works fine.
If I leave the USB cable connected and turn on the printer, I just get a blank screen. Obviously the processor in the printer is not resetting properly, but even if I push and release the RESET button (after turning on the printer with
the USB cable connected), the printer STILL will not start.

Things worked fine when I was using my original main board.

Does anyone have any ideas?

Don't plug into USB. Only use USB for firmware updates and printer settings.