Archispirostreptus death

[AnEndlessCold]

Hi guys,
I was devastated when I came home today and found one of my Archispirostreptus gigas dead. They were my favorites and I want to make sure I keep my remaining one alive and healthy.
I have her in a 20 gallon long with a 7 inches of a mixture of coco fiber, leaf litter, and softened oak wood. I use a 40w basking lamp to keep the temperatures up. I have hollowed out log half that they spent most of their time under, except recently the one that died had been curled up near the other half of the tank, out in the open.
I sometimes supplement their leaf litter with fruits and vegetables.
What can I do to improve this?

 

[BobBarley]

How moist was it? And what were the sub ingredient ratios? The basking lamp may have made the enclosure too dry.

 

[mickiem]

How old were they and how long have you had them? I never use heat lamps on my millipedes - they are very drying and generally hotter than they need.

 

[AnEndlessCold]

I've had them for about 6 months. I can do away with the basking lamp. The substrate is about 50% coco fiber 50% leaf litter and wood, but they hardly ever burrow and there's a layer of purely leaf litter at the top of the tank.
What heating device would you suggest, if any?