[nst7]

First, you need to make sure you're using a digital spidf cable, not a standard RCA cable, or you'll likely have problems.

And under Setup>Hardware, make sure Optical is set to ADAT, not Spidf. This ensures that Spidf is not trying to go through the lightpipe ports. Then select spidf as clock source, but again, make sure you're using the right cable.