I think I know why people are getting an error. Some of your assets are stored online, and not on the disk, so people who download the game and play it off their disk don't have all the assets. You need to play the game from the browser.
Viewing post in Mahō Shōnen: Magical Battler comments
Unless people are using the itch.io launcher (it should do a local copy of the game) everything is stored online.
What I did notice earlier with some people is that they get errors when saving, because they are playing the game on an incognito window.
Incognito mode doesn't allow the game to save data on the broswer, and that's when the game explodes.
No, I don't think they're talking about a save error. I got the exact same error they did in the exact same location trying to play from the itch.io launcher, and it had nothing to do with saves. The Loading error is not loading from a save, it's that the portal dungeons fail to load playing from the launcher.