I feel a bit stupid now, but I’ve confirmed it’s the cable. There is a short between blue and sync and a slight short (600-800 kOhm) to red.
When I buy cheap cables from China I usually measure them before usage, but I figured there was no need this time. Also, the symptoms seemed way too complex to be a simple short, still don’t understand how this wouldn’t affect all output more, shows what I know.
Anyway, thanks for all the help.