I would suspect some type of corruption with that folder. With OE closed, use the Windows search function to search for sent items.dbx (include system & hiddend files when searching). When you find the file, delete it. A new one will be created the next time you start OE.
Warning- all messages in sent items will be deleted when you do this, so if there are any you want to save, better try dragging them to another folder beforehand.