Oven cable melted cable and casing - can't work out why

[MFS Electrical]

The connection was insecure and of high resistance; the heat has melted the plastic housing and probably allowed the terminal to short-circuit to the metal casing. It's not clear whether your side of the connection was at fault or the internal one(s) hence Marconi's question. From the pattern of burning, I think it's your connection.

Common causes are crossed threads - the nut feels tight but isn't clamping the cable or cable not in position so that it creeps out from under the washer. Did you give it a good tug-test after fitting?

I agree and you can’t beat a good tug test