I have tried everything, mixing upper case and lower case (the way they actually appear in the game), and just typing them in, all lower case. It (the "Darkroom"), keeps accusing me of putting in fake, or "not real" codes. What nerve!

I just had this happen too. My solution was to re-evaluate what I thought the characters were. A lowercase "n" turned out to be accepted as an "r" and in looking back at it, yeah, I suppose their space-age letting style confuses how letters look. The letter "Z" actually turned out to be accepted as a "2". Hmmm,... Go figure.