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.)
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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