If you paste Seirdy.one into the URL bar and press Enter you get an error, "Proxy request refused".
Changing it to seirdy.one and the page loads as expected.
Note I'm not affiliated with this site, just run into the issue when performing those steps.
"Proxy request refused" tells that it's the server having issues not ignoring case, it thinks you're trying to proxy to another domain. However it's good practice for clients to lowercase domains before making requests, just incase the server is badly made (like here ;)