"unable to print 51" message error on Brother MFC 845CW

Solution:

- Machine off
- Switch on the machine while holding down the "Menu" button,  until all the leds are flashing on/off: it's the "maintenance" mode.
- Press 80
- Press the "Mono start" button several times until you see "purge: xxxx"
- Press "2783" in order to reset the purge counter. You will see: "purge:0000"
- Press the "Stop" button.
- Press "99" to exit the maintenance mode.