A solution was found at Microsoft KB article 233264
The result was a mixed success. The original error was gone, however, while the user's mail folders were intact, their hierarchy disappeared. This was a minor annoyance, as the user could easily recreate his folder hierarchy. However, a new error replaced the old one: "An unknown error has occurred. Account: '...', Server: '...', Protocol: POP3, Port: 110, Secure(SSL): No, Error Number: 0x800C013B
This was tracked down to Microsoft KB article 253795. The article advised to rename Sent Items.dbx file, which we were reluctant to do as we didn't want to loose the user's sent emails. Instead we created a new folder TempSentItems, temporarily moved all emails from Sent Items to it, then deleted Sent Items. When Outlook Express recreated Sent Items folder, we moved the saved emails from TempSentItems to it and deleted the temporary folder.