@chrishamm Update now that the new ribbon cables have arrived, unfortunately still the same issue. To ensure there were no remnants from before, I repeated the steps from before: set it up in standalone mode (all okay), updated the firmware, removed the SD card from Duet, installed a fresh copy of DuetPi, and then went through the setup process on the Pi 4. But I'm still seeing the same warnings/error messages as posted above. I'll go ahead and email warranty@duet3d.com as suggested.
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UPDATE: Email now sent