I have exactly the same problem as reported above. When I started my MAC some days ago, my old mails have disappeared and I cannot access the inbox from two account I use with pop3 via Operamail. The small little window in the left bottom says 'download mail and a number' but nothing more is happening. I receive emails from two imap-accounts, but as well, no old mail. It seems that I am not the only one?
What except reinstallation can I do?