Okay, I've managed to solve this myself.
As it turns out, the keyboard at boot is set to have NumLock enabled. Which, frankly, is kind of retarded.
So when I was typing the password 'admin', I couldn't tell since each of the five characters were obscured with '*', but MEBx was receiving 'ad05n'. Which is clearly wrong.
Quite frustrating.
So the work around, in case anyone else experiences this, is to turn NumLock off (forcibly) as soon as you enter MEBx. Note that I'm using a Thinkpad USB keyboard.