There's a few possibilities I can think of that caused your frog to perish. It is possible that the pH of your substrate is off (either to acidic or basic). You could perform simple pH test with the appropriate soil kit.

Another possibility is that you accidentally "cooked" him, many people usually over heat Tropical species of amphibians thinking that it's extremely hot and humid (which of course is not true). In reality the temperature of the forest floor in tropical rain forest is usually much cooler than one would think (usually in the 70's, hot air rises, cold air sinks. Plus much sunlight never reaches the forest floor). I would recommend low 80's at the highest for max temp, with upper 70's being ideal and high 60's-70 at night.

Your moisture/humidity sounds just fine, so no need to change that.

Last thought is you said you fed him supers? (I'm assuming that means super worms?) If that is the case, DO NOT feed super worms or meal worms! They can still maintain there ability to use their mandibles, especially in amphibians because they aren't chewing the arthropod to kill it completely. Plus, even with humans there has been incidences were individuals ate meal worms that they hadn't chewed properly and the meal worm ate a whole through there stomach wall. I would therefore stick to young crickets (gut-loaded of course), phoenix worms, silk worms and horn worms (the two later, b/c they have a very close 2:1 Ca - P ratio). Calcium is important in the Ceratophrys genus, this is because of their fast growth rate. IF they have inadequate levels that can develop jaw abnormalities or rickets (juvenile brittle bone disease) like syndromes.

Hope this helps! And sorry for the loss of your frog!