No login with letsencrypt certificate and hostname

Strange: I got a letsencrypt certificate and both “https://ip:9090” and “https://host.example.org:9090” lead to the server manager login page. But login fails under “https://host.example.org:9090” - wrong password. It only works using “https://ip:9090”. How is that? Any ideas?

Please check /var/log/auth for errors after a failed login.