We received replacement parts from Yarbo (THANK YOU!) and based on what we found taking apart the motor, that was definitely the issue.
The top of the belt pulley had completely melted into the motor and totally deformed. That made it difficult to tell exactly how the new part should be installed.
We weren’t sure whether the pulley should fit snug against the motor with the nut tightened (which made the motor spin with effort) or if there should be some room between the top of the pulley for the motor to spin freely. We are worried that installing it too tight or too loose will cause it to rub and break again and after extensive searching have found nothing that answers that question. Does anyone have any insight to this repair?