Pls have a look on this screenshots
Do you see any received messages if you check "papierkorb", "duplikate"?
Since this is IMAP though, you can just delete the IMAP account and add it back in and things should be fine. If not, you can close down Opera Mail, delete (or rename) the "mail" folder (see "Menu -> help -> about Opera" for the location), start Opera Mail and set up the IMAP account again. If you have any feeds though, you'll have to goto "Menu -> settings -> import and export -> export feed list" before you delete the mail folder and then import the list after you set up Opera Mail again. If you have any POP accounts, you can rename the "mail" folder instead and, set the POP accounts up again and then import your messages from the old mail folder (I'll explain how if needed).
Also, for good measure, you can close down Opera Mail and delete operaprefs.ini in the preferences folder "See "menu -> help -> about Opera Mail" for the location) when you delete the mail folder to clear any problems with the global mail preferences.
You can export your contacts and delete contacts.adr when you delete the mail folder. Then, you can import your contacts back in.