The article [Q295965] states that when OE is started, some existing folders (Inbox, Sent, and so on) or personal subfolders may be missing and can be caused if the file "Folders.dbx" (the Indexer) is damaged or corrupted. To resolve this issue, rename or simply delete the Folders.dbx file for the impacted identity, and then start OE which rebuilds it.
CAVEAT: The Folders.dbx file keeps track of Active News Group Subscriptions also. Should that matter, do not delete it. If you do, this information will be lost. Again be advised, the file Folders.dbx is the file which categorizes and maintains the default mail Store (in OE, click Options, the Maintenance tab, click Store Folder, and then make a note of the path) and default OE files. If a certain file or folder is moved or copied into the mail Store in lieu of using the correct Import method, the Folders.dbx would not be aware of this change and therefore consider the replaced ?.dbx file or folder as corrupted. To make sure things are in sync when copying or moving files into a mail Store, delete the Folders.dbx file before OE is started to ensure this new file is created accurately. Should the system have asked, "Overwrite . . ."? when the copy or move was performed, anything previously in the overwritten one was lost.