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.