I recently started to backup
Not sure how Symform does it, but in general, you should have any program touching Opera Mail's mail files while Opera is running as it can cause corruption of the mail files (the database for example). So, Symform should only be backing up the files while Opera Mail is closed. Even if Symform doesn't interfere with Opera Mail's reading/writing of of its files, backing up Opera Mail's files while Opera Mail is running will cause the files to be backed up in an incomplete state.
With that said, if you have a super-recent, *good*, backup of omailbase.dat, that'll probably fix it.