Well, I'm very sorry to say that I also have no idea what that means, haha. I'm very much not a programmer and only have a tenuous grasp of Twine, honestly. HTML stuff is beyond me.
My best guess is that there's an issue with your browser (I've only tested on Chrome and Firefox), or that you're using incognito mode/private browsing (understandable for a game like this!).
I tried loading a save using Chrome Incognito, and got the same error you did. Weirdly, Firefox private browsing seemed to work fine.
If you're using Chrome/Firefox, not using incognito, and still getting this error, then I'm afraid idk what to tell you. I'm out of my depth. Hope you can make it work, though!