This black powdery sheet (!) is not burnt paper, but, paper eaten by fungus.
Cellulolytic microorganisms are hiding nicely all around us and are much more destructively powerful than we can expect from the tiny spores.

Storage enclosures that were designed and created for protection turned out to be actually working with the deteriogens (deteriorating micro-organisms), by providing them not just the ideal and extremely comfortable environment to propagate, but also all the food supply in the form of cellulose for their generations!