Good call on the cable, Jim (Jea48). I would not have guessed that. And your explanation sounds very plausible. I agree with Kijanki's comments as well.
Jadedavid, the reason that a length of 1.5 meters is often recommended for digital cables is further explained in this paper
. It relates to the potential effects on jitter (short term fluctuations) in the timing of D/A conversion, that may result from the inevitably less than perfect impedance matches between the cable and the components that are being connected.
As Kijanki indicated, though, and as can be inferred from the paper, many system dependent variables factor into the likelihood of that length being optimal. A notable one being the risetimes and falltimes of the signal that is sent into the cable (i.e., the amount of time for the signal to transition between its lower and higher voltage states, and vice versa), which are generally unspecified and unknown. So while 1.5 meters may generally stand a greater chance of being optimal than 1 meter, that will not always be the case.
All of that relates to sound quality, though, and usually not to the kind of problem you have encountered. But I wouldn't be surprised if the characteristics of the shield relate to it, as Kijanki indicated. Given that two different appliances cause similar symptoms, though, my guess is that it is not differences in shielding against RFI pickup that are involved, but rather effects of the shield or other signal return conductor in the cable on a ground loop issue that may be occurring between the two components. A ground loop issue conceivably resulting in the marginal lock Jim referred to.
An interesting experiment, therefore, might be to temporarily put a cheater plug (a 3-prong to 2-prong adapter, with the safety ground connection not connected), on the power plug of either the DAC or the component that is providing the signal to it. That would break any ground loops between the two components. Then see if the problem still occurs with the shorter cable.