Sounds to me like the male connector going into the female wasn't making a perfect connection, causing a higher resistance in the connection than normal.
This means that more current will be drawn through it regardless of what you motor and ESC is doing, consequently the temperature around the connection point increase to the stage where it melts the plastic.
I know this has happened before to others that have been running the Corraly style 4mm plugs and TP LiPo.
Ian
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