Yes. Turn low-bandwidth mode off for the account if it's on. If that doesn't help, make sure "do not download attachments" is turned off. Both can affect loading of messages. The latter even affects HTML messages.
What happens if you right-click the message and choose "Fetch complete message"? Does it load then?
You might try "make all messages available offline". Or, you might try setting "do not make messages available offline" first to clear the downloaded message cache and then change it back to "Make all messages available offline" or the the default "when I click them" and see how things are.
You could also delete the IMAP account and add it back in.
Or, you could log the connection to see if there's an error.
Far from me to even mention it, but your operating system does not sound to be of the most optimised of installations. Did you suffer from any of the problems you face now, before the re-installation/upgrade ?. Oh, and what exact version numbers (found in 'Menu /About Opera') of Opera were and are you currently using ?.