Fast Track C400 S/PDIF Loopback not working?

[Rualark]

Hi!

I use Fast Track C400. I connected S/PDIF out to S/PDIF in to record sound coming from S/PDIF out. I am playing sound to outputs 5/6 (S/PDIF) and recording from inputs 3/4 (S/PDIF) in Reaper (tried other applications too).

I tried two settings:

1. When setting Clock source to Internal no signal is recorded on S/PDIF (horizontal line).
2. When setting Clock source to S/PDIF i get a glitchy sound and error. Sample rate is constantly fluctuating somewhere between 47900 and 48000.
http://clip2net.com/clip/m32789/e7445-clip-61kb.png

Recorded sound looks like some segments:
http://clip2net.com/s/3gmxKZo

Could you please help me understand how to set up loopback on C400 properly? Thank you very much in advance!