OK. Speaking as a EE..
You've got the Tesla Mobile Connector. One end is in the car going to the adapter part with the electronics in it; the other end is a plug, stuck into the wall socket.
So, let's think about this a minute. There's an error on the screen: "Charge Rate Reduced, Wall Plug Temperature High."
So.. guys.. exactly how do you think the TMC knows that the Wall Plug Temp is too high? It doesn't have a finger.
Implication: The adapter end that plugs into the wall socket has a thermal sensor in it, courtesy of Tesla. Could be a thermistor, could be a thermocouple, could be the right kind of diode; but, any old way, there's electronics in the thick part of the TMC that's measuring the temperature of that component.
My experience with the above named temperature measuring components is that, generally, they're pretty accurate, to within a few degrees C.
I don't know what temperature Tesla sets that alarm to; but I take a flying stab and say that it's likely to be around 100C-125C. And maybe higher. Let me check something.. OK, did a random Google search, found a NEMA5-15 socket by Schurter. Operating temperature range: -25C to 150C.
So, my bet of the day is that Tesla's hardware/software is tripping around 110-125C, down in the guts of the socket where the metal blades go.
The next step past this: Let's Have Fun With Fire!
As a previous poster stated, this can ruin your whole day. And maybe your whole house.
If you get an error message like this.. I'd say, Stop Charging And Get It Fixed, ASAP!