Thanks for the reply. New development: I connected the N64 to a display using S-video and it works perfectly. So, the console is outputting luma. Could the problem be that the console isn’t outputting R and G and B? Or is the problem probably just the scart cable?
Related question: Can I trust this OSSC, since it upscales my Genesis signal perfectly? Or is that ability unrelated to the ability to upscale N64 signal?