The preview pane is enabled but does not show email.
It only shows the sender(s) and reciepent(s).
In Outlook Web Access (OWA) click on Options, and look for email Security.
There you'll find the option to install or reinstall S/MIME-controls.
Click it and install the controls. After the install is finished, restart Windows Internet Explorer.
Browse back to OWA, and the preview pane should function as expected.