Today I noticed that none of the codes that I had picked up for accessing different wizardlands seem to be working; They all go to Diggle Hell, including the ones that I haven't used before
That's... strange, that should only happen when a typo is made. If you have a code you've been to before it'll tell you in another way.
Did you use the trick to reset Wizardlands before this? I've done it once before but it hasn't affected anything to do with the portals, which work as advertised and only lead to Diggle Hell in case of typos.
UPDATE: I just tried accessing wizardlands again with some other codes that I found and it seems to be functioning again. My new theory is that the game is working but the .txt file I was writing down codes in somehow got corrupted.
No, this is seperate from the non-reuse thing. I've used that exploit in the past to successfully reuse wizardlands (and have, in fact, checked by reusing the wizardland that I used when it started working again). This is different, and I'm starting to think that it isn't actually a game issue after all. Like I said i currently think that what happened in this most recent case is that the notes I took down got garbled, like perhaps the last time I referenced them my mind was still in game mode and I absentmindedly tried to scroll down using the S key instead of the Down Arrow, or something, thus accidentally inserting extra characters into unknown positions in my notes. In short I think this one may have actually been human error on my part.