I also measured SIN and SOUT on both IC's, both boards, giving same pattern (digital tiny flicker around 3.1v).

I checked the LEDS to determine their internal wiring. Found that 3 corner leads are common anode and 3 remaining are cathodes for R / G / B. When I apply low voltage with multimeter in Diode test mode, on good board I get full R / G / B light on each pin, but same test on bad board, I barely get any light (very dim) and sometimes it seems colors get mixed even when Im carefully with leads.

Could be a bad diode, but also the shortcircuit could be in the feeding driver IC (TLC)

At this point Im stuck between bad diodes or bad driver. I guess I should remove one LED and test it isolated, but Im afraid Im going to fry the board as I don't have any good and thin solder tip neither any special tool to desolder 6 pins at a time

Any other hint on how to diagnose?

Thanks!