The locally cached copy of the folders in question may have become corrupt.
Use the following steps to clear the local cache of an individual folder and force a re-sync from the Exchange server:
1. Within Outlook, right-click on the folder.
2. Click on Properties.
3. Click the "Clear Offline Items" button.
4. Click OK on the confirmation window.
5. Click OK on the Properties window.
6. Click on the Send/Receive tab.
7. Click Update Folder.
Outlook will download the contents of the folder from the server. Note this process may take a few minutes if there are many items in the folder.