Index.html suddenly not working. Couldn't load scenes/startup.txt

I’m getting the error message “Couldn’t load scenes/startup.txt The file is probably missing or empty.”.
I have tested it with the other wips that I previously had no problems with – they are free of typos. But suddenly the new wip and everything else gives me the same error message.
And no, I’m not using safari, I’m using firefox, as recommended.

The error started happening since I accidentally opened unittest.js, but I haven’t touched the code in that file.

Edit: Trying playtest on dahsingdon works, so it’s not the scene file that is the problem.