Thermal Runaway can mean 1 of 2 things:
- Your nozzle is way hotter than the firmware expects (highly unlikely)
- Your thermocouple (the thing right at the nozzle) have an issue.
The firmware reads the temperature quite a few times and then average this to display, however if one of those reads produce a temp higher than the maximum set in the firmware, it shuts down and show the thermal runaway (the printer thinks its lost control over the nozzle temp).
On mine this error was caused by the Thermocouple Amplifier (which is located inside the black pipe right above the head) – if the wire is loose or fails to make contact, the amp will report a very high temp.
The previous firmware was a lot more lenient regarding this, but this firmware is very strict (in that it will not wait to confirm if it was a read error – it just terminate). Ultimately there is an issue which is best to get it fixed asap.