I managed to fix it just after posting this. The problem was that the UNO's bootloader was corrupt - likely as part of my HIDUINO experimentation. I used the ISP to reflash the main UNO chip through the Arduino IDE. After that, it worked great.
I managed to fix it just after posting this. The problem was that the UNO's bootloader was corrupt - likely as part of my HIDUINO experimentation. I used the ISP to reflash the main UNO chip through the Arduino IDE. After that, it worked great.