Do you remember that previous log where I was talking about the SK6812 pinout? I was surprised to see two different pinouts for the same led...
When I did the first tests on the boards I've received, I couldn't succeed in programming them. The Arduino used as ISP was shutting down when connected to the board. I didn't have much time for further testing, so I put it aside for a few weeks. It appeared there was a short somewhere on the board, with ground and VCC being tied together. And it soon appeared that it was coming from the leds, because when not mounted I was able to upload the program.
And the problem was of course these leds' footprint. In fact there are two different one, one for the 5050-sized leds, and one for the 3535 ones. And of course they don't match, unless you turn the led 90°, but then the leads are not exactly matching the pads on PCB. So by turning leds around 80° instead of 90, I was able to test boards!

A new batch of newly drawn boards have been ordered today. With a few other minor corrections... Wait and see!