Is it any method to check if the strip is working, without the controller?
The external power adapters output is 19V, and I’m measuring 19V on the ledstrip’s soldering points, but 5V on the VCC/GND on the led itself. Could 19V have damaged something?
Normally controllers are same voltage in and out. Generally the only voltage conversion is to drop the input voltage to 3.3v for the Esp chip, not the LED output.
You may have gotten lucky and only killed the first couple LEDs on the strip, if you want to try connecting your data a couple LEDs farther down the strip to see if they work, it’s worth a try.