So apparently the URL for changed. Sadly the new one makes the same old mistake by sending the _wrong_ Content-Type making my reader trying to parse it as JSON feed. Proxy rewrite it is again *sigh

For nginx this could look like this:

location /fds-nina-feed {
    proxy_pass https://warnung.bund.de/api31/mowas/rss/082370000000.rss;
    proxy_set_header Accept "application/rss+xml;q=0.9,image/webp,*/*;q=0.8";
    proxy_hide_header Content-Type;
    add_header Content-Type "application/rss+xml; charset=utf-8";
}

Find more broken feeds at https://warnung.bund.de/meldungen