It must be Wretched Wednesday — the day after Black Tuesday. Watch out for automatic patches KB 2817630, KB 2810009, KB 2760411, KB 2760588, and KB 2760583
This behaviour seems to be caused by using the latest version of the shared Office library mso.dll 15.0.4535.1002 from KB2817630 in combination with an outdated version of Outlook. Try installing KB2817503 to update Outlook to the matching version 15.0.4535.1004. I don’t have the issue with the missing folder pane in this constellation. Updating Outlook with the August 2013 hotfix (KB2817503) restored the reading pane on multiple machines.
KB 2760411, KB 2760588, and KB 2760583 are parts of the MS13-072 and MS13-073 security patches for Office 2007. There are many reports of the patches being offered and re-offered and re-re- … you get the idea. On sevenforums, poster harin35 says: