Posting on behalf of someone else:
I had an australian land hermit crab recently pass due to some type of bacterial disease ( likely black spots/shell rot ) and this isn't the first time it's happened, it's also happened to a friend. It can vary to just the crab being active but then dropping dead the next day, to a crab leaving shell, not getting back in and dying overnight.
I've had many different setups, and all the crabs I've owned had the same fate.
The crabs seem to get a black line like pattern soon all joining together into a large oval on their abdomen.
Current tank:
42 gallon, front opening tank with a large heatmat at the back, the crabs have two available fully submersible dechlorinated water dishes ( salt water and fresh water ), a food bowl, hiding places, climbing places and foraging materials.
The hermit crabs have a diet of nuts, vegetables, foraging materials, coconut, mealworms and sometimes fruit.
The tank is usually 27°c and 80% humidity. The tank has only 5 inches of Coco substrate and sand mix due to moving.