MKR Motor Carrier: Firmware and Connection

Official Hardware MKR Family MKR Shields
1

When using the MKR Motor Carrier with a MKR Zero I'm notified that the connection fails:

Couldn't connect! Is the red LED blinking? You may need to update the firmware with FWUpdater sketch

The red LED is not blinking but since I couldn't find any more information on what to do I tried flashing the firmare:

10:46:20.320 -> Erase flash
10:46:20.815 -> Starting flash
10:46:21.657 -> 1
10:46:21.657 -> EF
...

However, it goes on and and on just outputting

10:46:21.657 -> 1
10:46:21.657 -> EF

for many minutes. What can I do?

2

I adapted the code of the firmware flasher to simply print the current version of the board:

#include "Wire.h"
#include "ArduinoMotorCarrier.h"

void setup() {
  // put your setup code here, to run once:
  Wire.begin();
  Wire.setClock(1000000);

  Serial.begin(115200);
  while (!Serial);
  pinMode(6, OUTPUT);
  digitalWrite(6, HIGH);

  int prev_address = -1;

  Wire.beginTransmission(0x66);
  Wire.write((uint8_t)GET_VERSION);
  Wire.write((uint8_t)0);
  Wire.endTransmission();

  Wire.requestFrom(0x66, 5);
  Wire.read();

  String version = "";

  while (Wire.available()) {
    Serial.print((char)Wire.read());
    //version += (char)Wire.read();
  }

  Serial.println();
}

void loop() {
  // put your main code here, to run repeatedly:

}

However, communication doesn't seem to happen either:

10:00:08.671 ->

I reseated controller and carrier but to no avail. Can I suppose that the carrier board is broken? There are no error indicators whatsoever on any of the boards...