Having dealt with similar stuff (RS-485) in a previous life, I'll offer the following:

Transients on those long cables will kill the bus transceivers, and EMI protection was probably too costly to implement. Take the 3 affected boards and find the 422 connection points (physical bus to transceiver IC). Then identify the driver ICs, remove, replace and test.

FWIW, I replaced the 1488/1489 RS-232 pairs in a TON of packet modems over the years, and my old employer's repair depot did similar with anything connected to their product's backplanes. If you want I can help identify the ICs in question.