On the Original Prusa MK4/MK3.9, the I2C bus is used for the xBuddy board to communicate with other printer components. The components have different states, including a busy state: if this particular state occurs while data is being sent from the xBuddy board, the printer will show the message: "HAL detected an I2C busy when sending data via I2C."
Error name: I2C Send busy
Error code: #13312 (MK4) #21312 (MK3.9)
Restarting the printer is expected to resolve the error, and this is not expected to occur again. In case a restart does not resolve it, please contact our customer support, we will take it from here.