You just answered your own question, I think.
When the juice gets old(er) it changes color and something happens to it with the constant heating from the coil (I'm no chemist or scientist, either). I get the same sensation, which tells me to clean the tank, change for fresh juice, all the same things you did but without the dry burning. In my experience it doesn't matter what tank, it's all the same with this kind of thing happening.