An alternative pluralization of boxes. In 60-80s hacker lingo (when hacker ment something very different, btw), it originated from vax/vaxen (the, at the time, correct way to referr to two or more vax computers). Because of this unusual pluralization, it because common to use -en instead of -es or whatever else was correct for all words ending in x or with an x sound, such as soxen or socksen for more then one sock. Box was/is usually referring to a computer (pc, server, workstation, whatever).
Our server farm runs on 25 boxen.