Error when connecting to ESP8266 on external power

Hi

When connected to my computer everything works fine. I see the green LED flash twice. But when connected to a USB power supply the ESP8266 connect callback gets called with an error object, and the red LED is turned on. Any idea why?

E.on('init', function() {
  SPI2.setup({baud: 3200000, mosi: B15});
  Serial1.setup(115200, { rx: B7, tx : B6 });

  var wifi = require("ESP8266WiFi_0v25").connect(Serial1, function(err) {
    if (err) { digitalWrite(LED1, 1); throw err; }

    setTimeout(function () {
      digitalWrite(LED2,0);
      setTimeout(function() { digitalWrite(LED2,1); }, 500);
      setTimeout(function() { digitalWrite(LED2,0); }, 1000);
      setTimeout(function() { digitalWrite(LED2,1); }, 1500);
      setTimeout(function() { digitalWrite(LED2,0); }, 2000);

      setTimeout(function() { wifiResetConnect(wifi); }, 4000);
    }, 1500);
  });
});

How do I debug this further?