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.
A fingerprint is a unique identifier of a specific version of the firmware. The Original Prusa XL checks the fingerprint of the puppies, to ensure the firmware flashed on the peripheral boards is compatible with the printer, and the boards are safe to use. In case this procedure did not occur in the correct time window, the printer will show the message: "Waiting for fingerprint timed out".
Error name: Puppy fingerprint timeout
Error code: #17521
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.