Over time the fasting screw could have became loose causing the connection point to arc causing heat and melting the connector.
As no other damage has occurred, I would assume that this was the case. And tightening all screws from time to time should a normal routine as vibrations cause screws to become loose.
(Unless the use of loc tight thread sealant is used)

When I received my 3D Printer experience told me the power supply wire diameter included was not capable of supplying the current the Controller, and neither was the 120volt AC Power cord.
The first Mod I made was to increase the wire size supplying the controller with ample voltage and current.

I chose flexible wiring from a Mobile Business radio that is used to supply power to a high power Business Radio Transceiver (10awg stranded).
As I have lots of varying lengths and wire gauge on hand from installing/uninstalling Mobile Radios, could not see throwing it away as wire always comes in handy.