Matrix is just relying on libcurl from PHP, so a reasonable test is to try curl from the command line on the server. This won’t always go through exactly the same settings as Matrix due to environment variables configuring proxies and potentially statically compiled libcurl in php, but it’s always a good first step.
Locally I get a reasonable response:
$ curl -sv https://blog.puc.edu/feed/ 2>&1 > /dev/null | grep '^< '
< HTTP/2 200
< server: nginx
< date: Tue, 08 Mar 2022 22:32:00 GMT
< content-type: application/rss+xml; charset=UTF-8
< strict-transport-security: max-age=31536000
< vary: Accept-Encoding
< x-nananana: Batcache-Hit
< x-hacker: If you're reading this, you should visit automattic.com/jobs and apply to join the fun, mention this header.
< host-header: WordPress.com
< vary: Cookie
< last-modified: Tue, 08 Mar 2022 19:59:49 GMT
< etag: "154c92d9452c60e719d3395ce619dfb9"
< link: <https://blog.puc.edu/wp-json/>; rel="https://api.w.org/"
< x-ac: 1.syd _atomic_bur
<
AFAIK certs are not cacheable so my guess would be either the server or a proxy it’s configured to use are resolving something different for blog.puc.edu than what public DNS is resolving. Could be something else, but curl should give some useful information in most situations.
