wrote:
Never mind I figured it out.

I have two of these motherboards, I went to the brand new one, the backup motherboard.

Both are supposed to be second hand but looks completely new.

The black square thing is indeed as I suspect some kind of plug that needs to be pulled out.

Apperently it's to protect the SPDIF optical port from dust/damage or so.

The one of the backup motherboard comes out real easy.

But for some reason the one on my main motherboard is stuck.

Perhaps glued or fried ?! Not sure.

But I will try and pull it out... maybe it can't come out cause of the i/o shield/bracket blocking it...

Hmmm...

Weird... manual says nothing about this.

Bye for now,
Skybuck.

That sounds like TOSLink. An optical fiber connector with
a large aperture (unlike communications fiber with a small
aperture). The optical fiber is also referred to as dental
fiber optics, presumably because in dentistry it is used as a
light source for something.

During PCB manufacturing, if you want the board to be
automatically soldered on an automated line, connectors
which can get "dirt" in them, receive a rubber plug. Once
a board is soldered, the water-soluble solder paste is washed
off in a washing machine. To avoid the wash water going into
the connectors, the connectors have plugs placed in them to
keep the water out. I had an RJ45 once, where the wash water
got in, and it left an insulating "film" on one of the RJ45
contacts. Inserting and removing a connector a few times
cleared it.

Optical connectors can be pretty sensitive to dirt too.

The rubber plug you see now, might be considered a "dust cap".
It helps keep the connector clean.

See if the "shape" of the connector, matches this.

https://en.wikipedia.org/wiki/File:TOSLINK.jpg

When TOSLink is operational, the color of light is
visible red light. The light source costs about a dollar.

The motherboard is NF4SK8AA.

https://i.postimg.cc/NF2xVYtH/toslink.gif

Paul