[flommer]

One obvious thing is that your spdif ports can't be used anywhere else in the session.

I'd also take a hard look at the Setup>hardware page. Optical format should be ADAT..

clock source could possibly be either internal OR SPDIF(RCA).. Try both..

I'd also test the thing using a send/return via a couple auxes or something just to see if the "hardware insert" aspect of it is causing a problem..