I definately agree with what Afro has said about the overcrowding. Stimpsons are okay in pairs but never 5 together.
I also agree with TrueBlue about the cat litter. The dust from that stuff is BAD. Their crap always gets hidden underneath and their urine stains it, and this WILL lead to scale rot when temps are low. And with 5 snakes crapping, and the snakes being exposed to each others crap isnt good, even though you are cleaning them out regularly.
Another concern is that you say there's 2 vents with the thermostat between them. Why do you need 2? I think you may be losing too much heat because of it and perhaps the thermostat isnt working the way it should because it's between 2 air vents.
Also, your cage is 1m high, right? Where are the snakes spending most of their time? If its on the ground then i would expect them to get scale rot and a respiratory infection. I cant imagine the heat getting all the way down to the cage floor.
Also, be aware that hot rocks can cause burns, especially during their opaque stage of shedding.