Thermal runaway in resistive elements
It sounds strange a solid state relay (on-off) controlled by a 4-20 mA signal. Anyhow the supply voltage should not damage itself the heating resistors, it looks like the internal temperature of these elements is over 700ºC. The resistance about 80 ohm, is it at cold temperature? Probably at 600ºC the resistance is much more and could explain 200 ohm. Try to check before the open/short issue comes up, the resistance of elements. If you can measure voltage and current, it is possible to calculate parallel value.