What happened?
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.
How to fix it?
Download the firmware, 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.