Hi there and welcome!
The MES' are known to have 2 problem major areas that cause them to fail.
1. The electrical connectors on the wires rot/corrode away at the heating element and/or the safety rollout limit switch. If they fail at the safety rollout limit switch the switch can burn up cause it is partly plastic (I think this is your issue)
2. The controller simply fails causing lack of heat control, the controller to not turn on, buttons to fail, or the whole thing to shut off
You could have a failing controller, you could have a failing safety rollout limit switch cause of connectors of cause the switch melted (my money is here), or you could be having a ground out issue.
The first and easiest thing to check is the electrical wire connectors. Your MES looks like a newer model so on the back midway up there should be an access panel that leads you to the safety rollout limit switch (this switch cuts power off if the smoker gets to hot... its a safety mechanism).
The other panel is on the back towards the bottom and that leads to the heating element.
Get into both panels and inspect the wire connectors UNDER the insulation and see if they are corroding. Give them a good little wiggle and tug and if they break then or they already look horrible then that is problem #1.
If at the safety rollout limit switch the switch has shown signs of burning, melting, or scortch marks then that is problem #2 but you can just disconnect the wires and splice them together here to bpyass the switch until u can replace it (if u want to replace it).
In any case you replace the rotted female spade connectors with these hi-temp stainless steel ones here:
If you replace any connectors you may as well replace all at the element and the safety rollout limit switch.
If the limit switch is failing or you bend/wiggle the tabs on the switch know that it WILL fail at some point after cause they are fragile here is the replacement for the switch... I highly recommend you get an order of these and order the EXACT ones I am posting as there are other cheaper ones but they do not fit properly. It's good to have a few cause they are delicate and any wiggle of the tabs from the back will guarantee a failure in the future at some point.
Start there and if that isn't the issue and the fix then we can talk about your controller. Your options are to get a replacement from
Masterbuilt or we start talking a simple rewire and using an Auber PID controller which makes your smoker 1000x better than what it ever was brand new haha.
Let us know what you find man and we will get you going cause the beauty is that the MES is not a super complex electrical device so we can beat it into submission so it works for you :)