version: 14.0.13
When using embedded mode, vaadin is using guest host ip/port for upload/download requests.
Chrome console error is
vaadin-bundle-8df68656d6e42345fc2b.cache.js:2496 POST http://host:port/VAADIN/dynamic/resource/2/0aa97b54-2607-43af-a516-9718a8adf927/upload 404 (Not Found)