@jimp:
If you haven't yet registered your SG-2440, do so at https://pfsense.org/activate/ and you will get access to https://portal.pfsense.org where you can download the factory firmware for that device, which as cmb mentioned is labeled ADI.
Once you register you can also enter a ticket in the ticket system and we can give you a link to the specific file you need if you are having trouble locating it.
Thanks - I had an SG-2440 that I ordered in May (I think it had 2.2.2 on it), and finally set up, and it had this issue. I tried to upgrade it via the console (it was able to access the internet, but not serve its own GUI website), and it still had this issue (I think another post identified the missing gid "wheel" associated with the 503 website error)
After registering the S/N (and renewing my pfSense gold membership at the same time), I was able to download the "ADI" memory stick version and burn it to USB, then boot to that version, and run the initial configuration.
Thanks for building it with a mini USB console port, and the console-friendly installation. That was really handy in this instance, as I don't know if SSH would have worked, or worked as well for this sort of re-installation task.