Retroarch Webplayer - SyntaxError: JSON.parse

Description

When installing RetroArch web player following these instructions and accessing the webplayer URL, these error messages are displayed:

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
onreadystatechange https://yuno.dev/retroarch/browserfs.min.js:9
    i https://yuno.dev/retroarch/browserfs.min.js:9
    _requestFileSync https://yuno.dev/retroarch/browserfs.min.js:6
    b https://yuno.dev/retroarch/browserfs.min.js:6
    setupFileSystem https://yuno.dev/retroarch/libretro.js:98
    afs https://yuno.dev/retroarch/libretro.js:44
    b https://yuno.dev/retroarch/browserfs.min.js:5
    e https://yuno.dev/retroarch/browserfs.min.js:5

Error: Unable to find listing at URL: ${listingUrlOrObj}
b https://yuno.dev/retroarch/browserfs.min.js:6
    setupFileSystem https://yuno.dev/retroarch/libretro.js:98
    afs https://yuno.dev/retroarch/libretro.js:44
    b https://yuno.dev/retroarch/browserfs.min.js:5
    e https://yuno.dev/retroarch/browserfs.min.js:5

Retroarch won't start and I cannot start a core

Version/Commit

You can find this information under Information/System Information

• RetroArch: 1.8.6 - from https://buildbot.libretro.com/stable/1.8.6/emscripten/

Environment information

• OS: debian 9 Strech
• web server : nginx
• Browser : Firefox 76 on Ubuntu 18.04

Start with the Emscripten setup instructions and trace setupFileSystem in libretro.js, including the BrowserFS requests shown in browserfs.min.js. Check the response being passed to JSON.parse and verify that the webplayer loads and can start a core without these errors.