Wareemba:
but now i tried to mount it on the outside of the fireplace - a cast iron box....
I put the k-probe tip in free air, it is fine, touch the fire box, wrong readings!
You have stated the thermocouple is touching the cast iron fire box, which is most likely grounded.
Remember that a thermocouple works by the dissimilar metals creating a voltage difference when heated. That voltage difference is amplified and linearized by the MAX31855.
There are two way the thermocouple can be wired to measure the temperature difference. Either true differential or single ended (where the low side of the thermocouple is connected to ground). I don't know which you have here. Either way, it is likely the high side is being connected (shorted) to ground by the electrical connection to the cast iron fire box.
What happens if you stick the thermocouple through the door and measure the internal fire box air temperature?