Oof, it's hard to remember it out of my head now, even though I made the game.
So, let me just clarify, is it inside the room/hallway that's unlocked by the "HollowEnds" password? If so, it could either be the boxes puzzle, which require no particular password, or the one close to the end of that area that requires you to input the number of holes on the ground?
I should possibly check the game files, in case you didn't figure it out by yourself, until now.