While the metal of your new V6 hotend can withstand very high temperatures, your thermistor can only go up to around 285°C for extended periods.
Set that as your maximum temperature by changing the first highlighted line to: #define HEATER_0_MAXTEMP 285 (or adjust the line corresponding to the hotend you're changing.)
#define HEATER_0_MAXTEMP 285
The Max settable temperature on the LCD is 15 less than the max temperature, you will need to set the max temperature to 300°C in the firmware in order to hot tighten at 285°C.
Remember to change the max temperature back to 285°C after you have hot tightened.
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