I’m sure you’ve already checked, but…
Have you checked the routings in the Audio/MIDI set-up menu?
Are you sure that it is an optical problem and not an audio problem (audio routing is correct, speakers are on and properly connected, etc.)?
Are you using the original, MKII or MKIII?
What DAW are you using?
Nor sure about MOTU and the 828, but some units are finicky if you don’t use both the optical in and out.