That is confusing, MXLinux is Debian as much as AVLinux is Debian, i.e. they are both closer than Ubuntu, but they are not straight Debian from the Debian repositories. They have customizations made, so just refer to the distributions by their names, MXLinux, or AVLinux, or actual Debian. No need to make the situation even more confusing.
But, to your current question, there is this note in the user manual:
“Power User Tip: Remember to set “S/PDIF” as the Clock Source and set the sample rate to correspond to the external device in the AudioBox VSL Setup Tab (Windows) or in Audio MIDI Setup (OS X) when using the S/PDIF input for external sync”
That implies to me that you have to both select S/PDIF as the clock source and set the sample rate correctly, that the interface will not force the sample rate to match the incoming clock rate.
In fact, later in the manual I found that it states that explicitly:
“Power User Tip: When slaved to an external clock, the AudioBox 1818VSL will not automatically change its sample rate to match the external clock. As a result, it may fail to sync to the clock source. If your AudioBox is not syncing to an external source, make sure that both your master device and the AudioBox 1818VSL are set to the same sample rate.”
The Motif XS8 manual says that the output is 44100, any chance you set the session sample rate to 48000?
Do you get indication from the sync light that the clock successfully switches from internal to S/PDIF sync?
"If you want the AudioBox to receive sync from an external device, choose the digital input to which the external device is connected (S/PDIF or ADAT). The AudioBox 1818VSL’s sync light will flash from blue to red. When the AudioBox is in sync, the light will be blue. "
I do not know if ALSAMixer exposes this, but the manual says there is an option to re-route main audio to inputs 9-10:
Main to S/PDIF In: Bypasses the SPDIF Input and Routes Main Mix to DAW Inputs 9 and 10.
Seems you would definitely want that not selected.
And please excuse the overly basic question, but is your interface mounted in a rack or somewhere that is difficult to see the back? Did you make sure you checked in good lighting to verify the connection from the keyboard was actually to the input connector, and not accidentally moved over to the output connector?