Well there is an issue and until you figure it out you need to get your snakes out of there ASAP, if the plastic melts this means the temps are very high and obviously way beyond safe, you could lose your animals even worse your house if something is defective overheat and could possibly catch on fire.
Check each animal one by one, BP even if suffering from a burn would still spend time on the warm side and keep cooking if you let them.
This is a very serious!