Hello!
Thanks to Chris I finally had my display working completely!
I changed the whole board. It had quite a few bad solder joints as expected of course. It didn't work first time, silly me, I had missed a few more bad solders (no stop, no repeat—I had focussed on the leds). My original PCB presented the same symptoms, even if it was one of the last produced.
Thanks again for your support and invaluable advice.
Jacques