I'm not sure what it is, as I'm not experienced with either scale rot or burns, but if it is scale rot or a burn, it looks fairly minor to me. Hopefully someone with more experience can help more.

How deep is the substrate? I believe it should be only around 1/2" deep or so. What is the temperature directly on the glass above the heat mat (with no substrate)? If the temperature is above 94F directly on the glass, then you need to adjust the thermostat so the temp above the heat mat stays 94F or below. They are fine with a basking temp as low as 88F, so even with 1/2" or so of substrate, you should be fine with lowering the temp of the heat mat a few degrees.

You also need to bump up the cool side temps - they shouldn't go below 75F, but 78F and above is better. As for humidity, have you tried a larger water bowl or placing the water bowl on the warm side? I've also heard that people have good luck with adding sphagnum moss to the tank, or adding a humid hide.

I hope that helps!