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.
XLBuddy firmware includes all the needed files for XLBuddy and puppies, including bootloaders. In case an incompatible bootloader is flashed, the printer will show the message: "Puppy uses incompatible bootloader protocol #, Buddy FW requires #".
Error name: Puppy incompatible bootloader
Error code: #17515
The error message will include the bootloader version that is flashed, and the bootloader number that should be installed.
The firmware for all puppies is part of the Original Prusa XL firmware and bootloader files. Download the indicated official firmware and bootloader, and flash both files.