No reaction while runnig sample code in Django project

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I've set up an HTTPS web server using Django and Nginx.
With the sample code is in the repo.
After I select a file to compress, nothing happens — there's no response at all, and I don't see any messages in the browser console either.
To debug, I added console.log("ret", ret); inside the function callMain(args=[]) so I could see what entryFunction(argc, argv) actually returns.
I got: ret 6
What does error code 6 mean in this context?
How can I track down and fix this issue?