Back to the backup of your mail archive and try again. Once in a while, make an extra copy and compact the database. If it fails somewhere, go back to your latest copy.
You might see if there are OE recovery and repair programs available for the Mac. Those, most likely, aren't free.
I am having a major problem with Outlook Express on my wife's ancient OS9 Mac. From yesterday - having cleaned out and deleted hundreds of old 'sent' messages and deleted files I am now getting the message 'An error occured while launching - the end of file was reached'. I've tried a few forums and followed advice - rebuilding desktop, zapping PRAM etc etc - but nothing works. I now find that if I remove the database and messages files from the 'Main Identify' folders, express launches - but as if it were first installed - no contacts, sent messages etc. If I copy the files back to the folder I get the same error message. The database file is over 80mb. I (and she) would dearly like to retrieve the sent messages and old inbox messages as she uses it like a filing system. Any help would be gratefully received!