What happened?
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 overheated.
How to fix it?
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.
After that, restart the MMU unit and try the print again.