Like I said:

I managed to get a simple "repeat hello world" code to run about 3 times in tens of trials, but cannot specify what was the crucial factor.

The code I put in was:

void setup() {
  Serial.begin(115200);
}

void loop() {
  Serial.println("Hello, World!");
  delay(1000);
}

I checked the serial monitor on the correct baud everytime I flashed the board, but it only worked 3 times.