extruder temperature drops unexpectedly

Hi everyone,

I have a problem with my CoreV4 500 IDEX.
At higher temperatures, around 240°C in the extruder, the temperature drops and the printer shuts down. On suspicion, I swapped the temperature sensor from the right extruder to the left. Unfortunately, there was no change. I have also performed PID calibration several times without any change.
The point at which the temperature sensor fails is always different, but it never makes it past the first layer.

High temperatures of 260°C while idle are held constant for hours without fluctuations.

Printing PLA at 200°C is also not a problem.

I would actually suspect a faulty cable, but it is very unlikely that the cable from the left extruder has the same issue as the right one.

Does anyone have any idea what this could be?