Many thanks for sharing @wiztecy! First, really great to see a breakdown of the 400V controller regardless of the fault. The one terminal of R4 looks horrendous as you noted. Did you get a chance to pull the bottom PCB to see what the underneath of the board looked like? Maybe the solder contact trace is shot underneath although the resistor itself is fine (at least with no V going across it). Re the connection with water intrusion.The box seems well designed (minus a gasket). I doubt it could get enough water in there to reach the underneath of the board, but I guess anything is possible. As you note, the bottom PCB was not encapsulated. I often wonder about effects of severe condensation though. I've seen warm blasts of air come into my barn to make it rain off of cold metal surfaces.