How are you powering the Hosa splitter? And which model is it?
The three primary suspects: splitter box, optical cable, and D5 optical output. Easy one first: try a known-good optical cable. If that doesn't fix it, the problem lies either with the D5 optical output or the digital format converter (DFC) box. You can test for the D5 optical output by receiving its output with a device that has a known-good optical input (ifi you have access to one). You can test the DFC box by sending it a signal from a device that has a known-good optical output (if you have access to one).