WS2812B LED strip + Arduino First Pixel keeps dying/breaking
I finally found the solution:

The soldered pin connections at the cable was leaking current. This was due to the soldering flux remaining around the pins. Rinsing the connection with water carefully solved the problem. 

Just scraping, and wiping with tissues between the connections is apparently unreliable. The flux is water cleanable, said the cup. Of course by taking care not to make anything wet besides the connection, and drying careful afterwards. Perhaps it failed after a minute or so because the flux started flowing due to heating up. 

Thank you for your tips. You help me keep improving myself.

Any more tips on cleaning soldered connections are welcome.