On the Original Prusa XL, communication with the peripheral boards (e.g. Dwarf board) is achieved using the MODBUS protocol. In MODBUS there are two types of devices, the master and the slave (also known respectively as client and server). On Original Prusa XL firmware, the master is the XLBuddy and each slave is called puppy.
Each puppy is identified by the printer based on its type. If the puppy type is different than expected, the printer will show the message "Unknown puppy type".
Error name: Puppy unknown type
Error code: #17516
Inspect the wiring on the Sandwich board for a cable positioned into an incorrect port, a loose connection, or a visibly broken cable.
Download the firmware and if necessary the bootloader, and transfer it to your USB drive. Make sure to unmount the drive from your operating system before physically removing it.
Insert the USB drive back into the printer and reboot the printer using the reset button directly next to the knob to initiate the flashing procedure. In case the procedure does not start, reboot the printer again by pressing the same reset button, and while the printer is booting up, press once the rotating knob.