Just looking at the photos, its very obvious it cant possibly be sunburn..
absolutely no question..
there is no way the sun can burn both sides of a leaf, but leave the center of the leaf unburned..(top most leaf in the first photo)
that alone proves its not sunburn..
I cant think of anything else it could be except some kind of contamination in the media..
I would immediately repot..
whatever it is, its not likely to get better all by itself..
IMO, repotting immediately is the only way to save the plant..
if you do nothing, it will almost certaintly die..
actually, I would go extreme..
not only would I repot, but I would cut off ALL leaves and traps except for that one newest leaf emerging from the rhizome..if the contamination is "in the plants blood", so to speak, I think it would help to just remove all those "diseased" leaves..let the plant start from scratch as a simple rhizome with just a few new leaves..
remove it from the media..cut off all the old traps (except for that new one emerging) cut off anything questionable on the rhizome and roots,
swish the rhizome around in a bowl of distilled water, to really rinse it off well..then repot in brand-new media..not the same stuff you used before..
I would say the sand is the most likely culprit..you can never be sure what is in sand..
the peat is probably ok..
it *could* still die even you try that!
the plant is clearly sick..
it might already be too late, no matter what you do..
but IMO, repotting gives probably 50% odds of survival..
not repotting gives about 95% odds of death..
thats what I would do if it were my plant..
for those of you with aquariums..what is the first thing to do when you have a sick fish?
"do a water change!"
a clean environment is the first step to improved health..even if you dont know what the sickness is..
Scot