FATAL: cannot open /dev/ttyACM0: Input/output error
The reason seems to be that a buffer on the serial converter gets full and the chip gets upset and fails.
But you can easily workaround it. Disconnect the serial pins from the "emulator" (why do they call it an emulator in first place, I don't get it, it does not emulate anything) part on the board by removing the jumpers.
Connect the USB cable to your computer and now open the serial device in for example picocom or some other serial app. It should open ok now.
Now put the jumpers back in, you should get your serial output now.