Proxy-Server?

You probably need to configure the proxy server differently. ReactPress changes the PUBLIC_URL environment variable in package.json, maybe this needs to be accounted for.

Ah yes I do see that in the package.json. Taking that into account I updated the code to

let url = ${process.env.REACT_APP_PUBLIC_URL}/api;

? ? fetch(url)

However, the request gets sent to which does not exist…
https://(wordpress_site_domain)/wp-content/reactpress/apps/demo/build/api

Any other possible tips?

Proxy configurations are not my area of expertise. Maybe activate client-side routing. Otherwise, WordPress will ‘catch’ that route.

I will give that a try thank you. Also any new updates on the progress towards Vite?

Vite works now. If you follow the docs.