[aem-site-theme-builder] Something went wrong. Proxy error:

I've downloaded frontend theme sources generated by AEM for sites built with the [adobe/aem-site-template-standard](https://github.com/adobe/aem-site-template-standard) and [adobe/aem-project-archetype](https://github.com/adobe/aem-project-archetype).

After installing dependencies and trying to run Sites30 Proxy with `npm run live` I get a new browser window with just:

`[aem-site-theme-builder] Something went wrong. Proxy error: `

I should be redirected to the AEM login screen on the URL http://localhost:7002/content/wknd-spa-react-test.html?wcmmode=disabled.

I'm getting a 500 error in Chrome dev tools for the URL.

Reproduce the problem in the downloaded AEM frontend theme sources by installing dependencies and running `npm run live` for Sites30 Proxy. Inspect the request to `http://localhost:7002/content/wknd-spa-react-test.html?wcmmode=disabled` and its Chrome 500 response. Done means the proxy opens the AEM login screen instead of displaying the proxy error.

Written by the indexing model from the issue text.