I mentioned that it took several months to find this, as the symptoms led to a hopper error, so I exchanged the hopper with another machine, and after a day or so, 3100 came back. Sometimes the candle would not stop flashing after door open/close, which led me to focus on the door optics. Ran the test, and showed they were faulty, So I ordered new optics, and after a while, 3100 came back. Sprayed contact cleaner on all of the connections. After a while 3100 came back. Started spraying connectors 1 by 1 until 3100 was gone, which led me to the power connector. It has been quite a while now since the fix, and 3100 has not returned.
What was frustrating, is that the problem would go away whenever I started to chase it. I think rokgpsman might be on track, that perhaps a flaky ground or voltage may have been the source, and pulling the hopper in/out, and repeated open/close of the door may have jiggled it.