There is no exe file, as this is a web build.

This may be a very specific bug with Godot 4.3: https://github.com/godotengine/godot/issues/97434

This issue has only happened to one person who has tried playing my game on itch.io in Chrome and OperaGX. The game is not downloadable and is only available to play in the browser. The project file was not changed after I submitted it to the game jam.

The conclusion of the issue:

This sounds like a third party issue or a random error as it's occurring only to one person, probably something involving their browser or some setting or proxy

Without further details there's nothing we can do unfortunately, unless you can identify some specific thing about this user that is causing this, and even then this might be a problem in their browser or with itch that is out of our hands

So, it may be some very obscure issue involving your setup.  You should try playing the game on some other device/computer, preferably using a different network connection.  See what happens on your or someone else's phone/tablet/laptop/etc.