That's the first time I've ever heard of a snake with a yeast infection. Are you sure that's what it was?

I put humid hide boxes, when I use them, half on and half off the heat tape. Ball pythons in the wild are found in holes where the humidity is usually up over 90 percent, but you do NOT want them sitting on a damp surface. Use some kind of material in the box that will hold moisture but feels dry on the upper surface. Packed sphagnum moss or cypress mulch works well for this.

I'm betting that what you actually had was some kind of belly/scale bacterial infection caused by laying on a wet surface which doesn't have that much to do with the amount of humidity in the air.