VRT 392 Communication Error at Boiler

[Wonky Knees]

I recently swapped in a Vaillant 932 (wired, not wireless) (the user had a Drayton RF) on an EcoTec Pro. When the boiler and programmer are switched on, the VRT 932 displays "Com Error Boiler" when the VRT attempts to communicate with the boiler. The programmer doesn't communicate with the boiler at all, and the boiler runs continuously (default mode) even if the VRT 932 is switched off. Here's some other information that might be useful:

(1) The cable from the VRT 932 does not run parallel or close to any existing domestic electrical circuits.
(2) The jumper between terminals 3&4 is in place.
(3) I set the setback temperature to 5C (as some installations will run continuously at 15C).
(4) There are no fault codes displaying on the boiler.
(5) DHW runs fine.
(6) Reducing the HC flow temp. on the boiler (using the dial) shuts the fire, but the pump is still running.
(7) Turning the boiler off an on from the mains does not clear the problem; Turning the boiler off and on from the panel switch doesn't clear the problem, either.
(8) I observed polarity on the eBus (even though the installation instructions say I don't need to).

My hunch is that the problem is some place in the board, but I'd like your opinions. I want to make sure that I'm not missing anything.

Thanks.