If you decide to mess with Opera Mail again, there are a few things you can try. Here's one:
You can log the connection and look at the log. If Opera is getting stuck at a certain message or there's an error with a certain message, use a web mail or other client to move that message to another folder. Then see if Opera Mail fetches the missing messages.
Before you do that, close down Opera and rename the "mail" folder. See "Menu -> help -> about Opera Mail" for the location. Then, when you start Opera Mail, add the account back in (do not import anything and choose IMAP). That way, you'll be working with a fresh mail folder when testing.
If you get Opera to fetch more messages, but not all, examine the log again and see if there's another message that's causing a problem.