The wires or wire nuts of a thermostat are melted. What happened?
1. Heating wire in the electrical box, especially with single conductor wire. Remove the wire nuts and take the WarmlyYours wires off of the thermostat LOAD wires. Are the core wires of the heating roll the same diameter, or is one much thinner?
2. A bad connection under the wire nut. If the connection is weak, it will heat up and melt the wire nut and also the insulation on the wire. It happens over time, especially in areas where the moisture level is higher.
3. The unit may have been installed in a single gang box (2X4). Recommend the retrofit kit is the customer is in the US.