A while back I made a post about a comparison of different kinds of "coco" substrates.
I compared price, looks, performance and then added my thoughts ( also had pictures )
There were 4 different substrates. Repti-chip, Kritters Crumble fine (what you used) Zoo Med Eco Earth and what was formerly called Herptopia.
While I ultimately preferred Herptopia over the others, I figured I would mix all of them to use them up and from then on only purchase what I liked best.
The ONLY substrate I just threw straight out into the garbage was the Kritters Crumble. It has the "weirdest" and most unpleasant feel to it. Almost like super fine needles were in it. When you had some in your hand and closed your hand, it would prick you all over.
I have NO idea if that could have anything to do with your problem, but if the snake got "pricked" just right under some scales, it may have caused irritation and allowed a reaction. Possibly even allowing some germs to enter those spots.
All I know is that I would just about rather do anything then sit on a bed of Kritters Crumble. But then again, I don't have scales to protect me
edited to add a link to that comparison post I made. You will have to scroll down a bit on that page.
https://ball-pythons.net/forums/show...itters+crumble