The root cause of the issue was that I was using a relative URL for the baseURL. The Creative Editor SDK expects the baseURL to be an absolute URL.

So instead of

instance.addDefaultAssetSources({
    baseURL: '/public/cesdk/assets/',
});

I needed to use an absolute URL like:

instance.addDefaultAssetSources({
    baseURL: 'https://mywebsite.com/public/cesdk/assets/',
});

More information about serving assets is in this guide: https://img.ly/docs/cesdk/engine/guides/assets-served-from-your-own-servers/