I know this problem. Unfortunately, the only fix I know is reinstall (but it can happen again anytime).
Sadly, we cannot hope for a correction, due the circumstances.
Such a shame.. now everything seems to be working just fine, I mean I'm able to receive and send new emails, but cannot access the old ones. Luckily most of them is present on my email accounts' servers, so I can access them, but digging through hundreds of mails will be problematic...
Are you sure anything happened? If Opera thinks another copy is running, it will simply disable the mail client so that you can't have two programs writing the same file at the same time. And in Linux, the way it "knows" another program is running is by the presence of a lockfile.
Close Opera, then inspect both your profile and mail folders to see if there is a lockfile present. If there is, delete it.
For sure there was no other Opera copy running. As I said, I just casually restarted the system and then BAM, no old emails for me.
What do you mean by "lockfile" ? The file that is named like this somewhere in one of Opera's folders?