I think it strange the database hostname is not listed before the :1433 in your message. For a local database, you should enter either your real hostname, or
localhost, or the SQL aliases for local database which I believe are “.” or “(local)”.
Edit: if your SQL requires TLS encryption, it’s likely you’d need to use the real hostname so SQL’s certificate matches the hostname you use to connect, but I’d expect a TLS-related error message if that was the issue.
Also make sure SQL has the TCP transport enabled, under SQL Configuration Manager. The Root Server and Web Reports use the ODBC client, which can connect to local SQL using the default “Shared Memory” transport, but WebUI uses JDBC and needs the TCP interface enabled.