A collegue ran into an issue with Microsoft Outlook (2010) and the AVG Outlook plugin: when attempting to open mails on a shared Exchange mailbox, the message body would be cleared from that e-mail. The message body would be deleted from Exchange itself as well, resulting in other users accessing the mailbox to also see empty message bodies. If a mail contained attachments, they would be left alone.
After a long search, the culprit seemed to be the AVG Outlook plugin. Upon opening an e-mail, the plugin would scan the e-mail, which somehow went wrong, resulting in clearing the message body (probably because it was marked as bad?). Because of the nature of the mailbox (it's a shared Exchange mailbox), clearing the message body would then be synced to the server, resulting in empty mails for all users who accessed that particular mailbox.
If you run into an issue like this, be sure to check the computers of all users accessing the mailbox, and see if there's no antivirus plugin (like AVG) running.