well are u sure your thermometer is correct then? b/c i ruined my whole batch not to long ago b/c my thermometer was off bye more then 5 degrees i ended up opening them and they all looked pretty much like yours does.
I'm going to say the chick is day 6. And reason for death is developmental. The tail on the little guy is not proportioned right with the body, so I suspect other things are wrong with it's body plan inside. Nothing you could do about it.