Can't get Adafruit Neopixel to work with Wemos D1

Question

I am trying to get a very simple ESP8266 project to work but I keep failing and have no clue why.

These are my components:

• Wemos D1 (ESP8266MOD)
• WS2812B 5050 LED Stripe (2m with 60 LEDs/m)
• 5V 10A DC Power Supply

This is how I connected everything (I also tried multiple other pins on the controller)

Then I uploaded this code to the controller:

#include <Arduino.h>
#include <Adafruit_NeoPixel.h>
#define PIN            D2
#define NUMPIXELS      20
Adafruit_NeoPixel pixels(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);
int delayval = 500;
void setup() {
  pixels.begin();
}
void loop() {
  pixels.clear();
  for(int i=0;i<NUMPIXELS;i++){
    pixels.setPixelColor(i, pixels.Color(0,150,0));
    pixels.show();
    delay(delayval);
  }
}

Absolutely nothing happens. The LEDs remain dark. I measured the voltage directly on the strip connectors and it's fine. I've got no oszilloscope, but I get varying voltages on the ESPs data pin, so I suppose that's also working as expected.

I also stumbled upon this article stating that the signal voltage has to be at least 60% of the voltage of the stripe while my controller works with 3.3V (seee Datasheet). So I thought this might be part of the problem and tried it again wiht another power supply providing araound 3.8V at max. 5A with the same result. I am running out of ideas. Maybe you have got some for me.