Two things first off.
1)It's hard to help without actual thermometer readings.
2)If your snake is sick with a respiratory infection, take it to the vet. In addition to here, also check out the retic forum.
Now, your retic is always hiding in it's humid hide. That is an indication that there is either something special about the box, or something lacking in the rest of the cage. I'll bet it's a mix of the two.
Reasons for avoiding the rest of the cage.
a)to hot, the humid box is the only cool area.
b)to dry, the humid box is the only humid area.
c)to exposed, the humid box is the only retreat for security.
d)the unknown... only experimentation may discover this. Perhaps the substrate is abrassive, or non-existant, in which the snake likes something to grab. ie, the humid box moss or what not.
Reasons for staying in the hidebox.
a)see the above
b)it is the ample place to be the sit and wait predator that any large boid is.
c)it is the place in the enclosure that best provides the temp, humidity, and security needed for well being.
Again, you need temps. How do you know your humid hide is too cool, evaporation or not? Perhaps try including a hide at the warm end of the cage.
When all else fails, study the natural habitat, the snakes microhabitat/niche, and copy that to the best of your ability.
Ian