Just had this happen for a minute or two:
nginx error!
The page you are looking for is temporarily unavailable. Please try again later.
Website Administrator
Something has triggered an error on your website. This is the default error page for nginx that is distributed with Fedora. It is located /usr/share/nginx/html/50x.html
You should customize this error page for your own site or edit the error_page directive in the nginx configuration file /etc/nginx/nginx.conf.
So it seems like you have to look for a response status code of 502 (or 5xx) to catch this. The usual header of
Access-Control-Allow-Origin:"*"
is not sent with the error page, so there's a CORS blocking on a javascript fetch.