Port 993 + TLS for the IMAP server and Port 465 (or 587) for the SMTP server. Use your full email address for the username. As long as the server names and your password are correct then, it should just work.
While Opera is closed, you can edit accounts.ini in the mail folder and set Incoming Log File=path_to_text_file under the section for the account in question. Then, when you start Opera Mail, Opera will log the connection to that file. You can then see if it reveals exactly what's wrong.