Is it possible to review codes you already got? Because I already got a code that I need right now but forgot it.
The codes are randomized in this game, so a walktrough doesn't help and I can't load an earlier save game because I already overwrote all save slots with saves after I got this code.

It's been a week since you asked this, so you may have figured it out by now.

If you bring up your menu / inventory in game, there should be a "mail" selection in the lower left of the circle. I think all codes you receive in the game should be recorded there. It'll be listed as "Code 1", "Code 2", etc.

Well, unless it was from an object, then you would have to check it in your inventory. But I suspect it's not one of those that you're looking for.

Hopefully I'm not misremembering...