Just fixed this one internally. The reasons why it happened were a bit dumb, and fixing it created another even dumber bug that I thankfully fixed right after.
The bug will no longer be here in version 1.3, which I expect to release in the next 12 hours, and the Z ending will be accessible again.
Thanks for fixing it, and sorry for not catching the error code for you. Really fun game you got here.
There is one suggestion I would give, as I played I first thought you couldn't save at all, maybe mention in the tutorial about how to save? I know I figured it out at one point myself, but it felt a little hidden. Then again id say its short enough that do you need saves? It does get a bit hard on your first time.