Each of the three stepper motors on the MMU unit is controlled by a TMC driver on the control board. This message is shown in case the TMC driver for the Idler motor is almost overheated.
Make sure that there is sufficient airflow near the MMU board.
If you are using the printer in an enclosure with elevated temperatures, use ventilation to cool down the printer’s environment.
This is a warning and the print will be able to continue, but make sure to correct it or you might damage the hardware.