I have had the cat 5 from the HN board to a breakout board in the hall be the problem before. On that one it was snother mechanic crushing the wire when putting the plate back on after replacing a button. That car kept blowing the Pico fuses on the HN board. Not the same problem but I would definitely take 5 minutes and connect a different cat 5 jumper between the two to rule thar out.
