If the thermistor integrated into one of the electronic boards on the Original Prusa XL measures a temperature above 100 °C at any point, the printer will stop and show the message "MCU in  is overheated". In place of , the name of the electronic board that had the issue is indicated.
Error name: MCU Maxtemp Error
Error code: #17213
Place the printer in a slightly colder environment than the current one. For example, if the printer is inside an enclosure, place it outside the enclosure to help reduce the measured temperature from the boards.
In case this doesn't fix the issue, please contact our Customer Support.