We also had our first eggs die off this year. 8 egg clutch. All candled fine. Incubated same as every other year and all 30 other eggs hatched fine.
Three of the eggs started molding around day 42. But they still candled with veins, just no movement. When I cut on day 50 (bc I was way worried about the 3 very caved in moldy eggs) The hatchling were very under formed and completely dead. I assume they all died shortly before molding started happening. And most off the eggs were fill with a hard mass! Just like yours! The fourth egg died the day after cutting and other than being very little it looked fine. Very weird.
I am glad others are letting the general public know as well! Crap happens even when we try our best. And sometimes we will never know what happened...