Minimum length doesn't make any sense to me. Maybe I'm wrong, but I've definitely used 1-3 foot S/PDIF cables with no problems.I actually wouldn't be surprised if it's the cable. SPDIF (at least the Coax/RCA type) is actually fairly sensitive to the type of cable used, and oddly enough needs to have a minimum length (I think it's like 4 feet). Drove me nuts until I splurged for a decent set of high quality 6ft cables.
S/PDIF Coax cables are different from audio RCA cables, though. It's a digital signal, which is MUCH higher frequency than analog audio. It's easy for cable capacitance that's inaudible for a line-level RCA cable to screw up a S/PDIF signal.