SPDIF Ardour/Mixbus Motif XS8

It does look like all of the input channels are enabled and not muted, set to full input level.
I am out of ideas. I think you would have to find someone who both is running MXLinux and also has an 1818VSL interface to compare details of a working setup with your system.

Isn’t S/PDIF just the 9th and 10th inputs? Or did you mean it does not show the clock source selection when on the capture only page?

Yes spdif is 9&10.
I agree, you reached the same conclusion as I did.
It is not a big deal not having spdif, as I can just use two inputs on the interface and use XLR and an analog mixer, but I particularly prefer spdif for keyboards with spdif mixer. Makes life pretty easy compared to analog. I already look at the one trick pony platform where SPDIF works, I have it on another partition, but I could not find any info that could make it work.

Thanks anyway for the great help. You gave it your best shot and it was very helpful.