Okay, so @jcollins set me onto a path that made me (and the dev) realize that the problem revolves around attempting to hold the player’s place in the game.
As long as the browser stayed open and didn’t refresh (apparently, some browsers auto-refresh pages, and that was causing problems as well), then it worked fine. As soon as you closed the browser or the page refreshed, everything got hosed because all of the info in startup.txt (i.e., variables and achievements) were lost. Basically the game was attempting to continue while not knowing wtf it was working with.
I attempted setting up a mygame.js file in the “mygame” folder, containing all of the necessary game info that is stored in startup.txt in hopes that would solve the issue. The dev was convinced it would work, since that’s what the published game uses in lieu of startup.txt (mygame.js is actually generated based on startup.txt), but I couldn’t get it to work and still got the achievement error after closing the browser, opening it, and continuing the game.
So, in the meantime, to try to remedy this problem, he has removed the ability to “pick up” where you left off in the game. If you close your browser, or the screen refreshes, you’ll be put back at the beginning of the game. However…
You can use the local or server saves to get closer to where you left off. Now that we have the local save options working, you can save scum to your heart’s content. I recommend you do it often.
One caveat… when you go to play again, you will continue to get the achievement error until you delete all the data for your game and clear your cache/cookies for cogdemos.ink. I tested it and tried to get it to work without doing that, but kept getting the error until I deleted all my saves and cleared my cache. Once I did that, I restarted the game (at the screen where the error popped up), and was able to play without issue, even after closing the browser and loading my last local save.
I apologize for the aggravation with all of this, but I don’t see any other way to deal with the errors or get the game to pick up after being closed and still load startup.txt. At least now you won’t get bombarded with the errors. Just make sure to save.