Within an hour is really really fast, making me suspect something aside from fungal contaminant. I honestly can't imagine a fungal contaminant that would transform a healthy seedling to brown mush within an hour. Even in perfect laboratory settings on nutrient plates at perfect temperatures fungal cultures take 48 hours to really take off.
You mentioned diffused light but how diffused? Flasks are typically grown under low light conditions like shop lights. Bright light coming through a window, plus the heating effect of the window and mini greenhouse really seem like the culprits here.