Typical situation: the server only knows its own human-unfriendly corporate network name. But you access this server under a human-readable server alias, configured somewhere with your proxy, in the client DNS server, IP settings, in the user profile, etc.
So your browser can resolve this alias to an IP, but the server (and the applications running on it) cannot. It cannot connect to itself due to insufficient DNS configuration.
This may not be your case, but you can check if you log in to the server and try to ping it by the problematic DNS name or try to load the CSS URL from there.
As an experiment, try to replace the server name in the CSS URL with the server IP address or with 127.0.0.1. You can also add the DNS name to server’s hosts file
