The error is coming from HomeKit. I don’t care about HomeKit. Remove the Item from HomeKit (what ever that means, I don’t use HomeKit) or just ignore the error for now. It isn’t relevant. Make the Item work and then you can make HomeKit work.
What I need is to know what the JSONPATH is returning.
Keep the Item as a String. Post the JSON. And look in events.log to see what Temperatur is being changed to. If that doesn’t work, create a Rule to print out the state of Temperatur.