So you're saying that the same software and the same computer and the same cable work fine with another player.

But with this one particular player they do not work.

Assuming the same player-firmware-version is installed on the two players, then that leaves two thing as a possible issue: 1) there is a hardware problem, for instance, a bad ethernet port, or 2) there is something about the music/database installed on the player messing it up. Either way, sounds like contacting support would be the right way to go.

Unless it's something simple like you've used Hijack to block ethernet communications with the player or something...