Hi Jerome,
From your description and with the help of the picture you attached, I think it is safe to say that in your case, the problem lies on the 3rd LED strip from the right (the one with the top 2 LEDs not lit): the WS2812 controller chip that belongs to LED #7 (starting count from the bottom: 1, 2, 3, 4, 5, 6, 7, 8) is damaged and thus, data propagation stopped there. So neither LED #8 in that same strip, nor the other 2 strips to its right will light up.
Have you tried replacing just that 3rd strip with a spare one?
-Werner