toslink/s/pdif fussy

Depends on what they are connected to. What are they?

Yes, HDMI is a poorly conceived design. However, I don't see how this is relevant to the OP's problems.

Kal

Maybe if your source is not giving the signal when your D/A, or whatever is turned on, it may start looking for another source that has a signal. It may rule out that jack as not having its signal. If the source is on first, then the D/A may lock right onto it. Just a guess.

Also, there may be a short delay in the order the D/A search for a signal at turn-on. The Toslink jack your using may be the first one in the search sequence. The other inputs may be checked for a signal after the jack your using is found to not have the signal first. So the other jacks may give you more time to power your source up, since they (other input jacks) are checked for their signal after the one your using.

A little more. Your source may a little time lag on the optical out, in comparison to the coax out.

Or, your DAC/Processor just may identify the coax easier than it could an optical signal. If your source is powered on first, I would guess it may work better.