Digital IO Functionality on the SAMD21


I have a couple SAMD21 boards connected to a screen that I am trying to read PWM signals from screen touches from.
The two boards are identical - same components and same setup.

There seems to be some kind of short on one of the boards caused by connecting the screen. When I take the screen off, the problem goes away.
I set up an example, which just involves writing a HIGH signal to one pin and a LOW signal to the other.
So I took the two PWM pins (15 and 17) and connected them with a 220ohm resistor. On the good board, everything works and I read about 1.3V between the two (and about 2.4V with no resistor in between).

But on the bad board, this same setup with the 220ohm resistor causes the board to freeze entirely. Removing the resistor clears the error, but I shouldn't be seeing this at all, looking at the other board. The board setup is so simple - there are no components on the PWM/digital IO line. It is simply a straight line on the PCB from MCU to connector.
Is there an internal way to correct this? I'm having difficulty seeing a way to improve this simply between the pins.

Here is my example code:

void setup() {

  uint8_t _yp = 18; //PB08
  uint8_t _ym = 16; //PA05
  uint8_t _xp = 17; //PA04
  uint8_t _xm = 15; //PB09
  while (!SerialUSB);
  SerialUSB.println("Serial initialized...");
  pinMode(_yp, INPUT);
  pinMode(_ym, INPUT);
  pinMode(_xp, OUTPUT);
  pinMode(_xm, OUTPUT);

  SerialUSB.println("pinMode set successful");

  SerialUSB.println("Write xp high");
  digitalWrite(_xp, HIGH);
  SerialUSB.println("Write xm low");
  digitalWrite(_xm, LOW);



void loop() {

Thank you for any help!