Switch -> PoE injector -> MX2Wire Transmitter -> (~1000' of 2 conductor) -> MX2Wire Receiver -> Mobotix T25 Door station

^^ That's the setup. During regular operation, the T25 at the far end is powered by the PoE injector, as you can probably assume. When this door station went down, I assumed it was a faulty T25, but on the bench (directly into a PoE source), it tested out fine. OK, so now it looks like a bad MX2Wire set. Got a new set and the T25 still didn't work (this is when I thought the wire could be the issue). I ended up moving the camera from the closer location to this location, and that worked. Also, when I bench tested the 'bad' MX2Wire set with the camera from the closer location, it 'kinda' worked - I got screen shots, but couldn't get it to display a video stream.

So what I have is some parts that aren't fully broken, but they just can't do what they used to do. I never considered electronics to be at all like a mechanical device, which wears out and degrades over time, but it sure looks like that's what has happened here.