NAD processor issue, CEC problems.

The medium for transmission (optical) is not inherently flawed. No, NAD gear is usually reliable.

I would buy nice and durable cables - maybe even ones that look cool.

A cable (in theory) should not alter or improve the existing signal. If it claims to add more bass, reduce treble, and provide a more coherent midrange response....that cable manufacturer is lying to you.

Source components can make a huge difference. Cables should not have a sound signature. Think of 0 db - that’s what an ideal cable is.

.??? This is an hdmi cable which is connected to arc which can be unreliable.  Your comment is abit off topic mr master

What is happening?

Depends.

"...Depends..."

Thanks for that clarification. Good luck with your issue, I'm sure somebody will come by to help out.