It indeed looks like the message body for the message didn't download.
It does indeed sound like your mail folder is messed up and the stuff in the link will fix it. But, there are a couple of things you can try first.
If this is not happening in your IMAP "INBOX" folder, you can goto "Menu -> mail -> IMAP folders", unsubscribe from folder and resubscribe.
You could instead delete the IMAP account and add it back in.
If you just want to fix a few messages, you could use your webmail to copy the messages to another folder. Then, you could delete the originals and move the copies back.
Also, avoid using "do not download attachments" or "low-bandwidth mode" as those have downsides to syncing message bodies. Also, check on the "Incoming" tab in the account's properties and make sure you set the availability of messages offline to what you want.
But, just doing what's in the link will ensure that everything is right.
Regardless of what you do though, close down Opera Mail and edit incominN.txt in the mail folder (where N is the number for the account as seen in accounts.ini. You can goto "Menu -> Help -> about Opera Mail" for the location of the mail folder. Anyway, in incomingN.txt, you'll want to make sure Disable QRESYNC=1 is set to avoid synchronization problems. And, *if* you have to, set Force Single Connection to 1.