xasharma can you check the raw source of the page? You should be able to access it via the context menu by right-clicking on the page.
If the file is entirely blank, you need the URL rewrite workaround as described in Sitemap README file to make your webserver handle that particular URL via the Flarum router.
If the file contains XML, but also some javascript at the bottom, check your web firewall services and proxy services, you might have some spam or attack prevention script adding javascript to every request without checking the content type first.
If it's neither, it's possible your browser shows that red error because of a browser extension. In that case it's not related to a Google issue.
You should try upgrading to Sitemap 2.0 if you can. None of the modes are impacted by the 50k file limit in 2.0
