- Go to your Exchange Server 2010.
- Start the Exchange Management Console
- Navigate to Microsoft Exchange > Microsoft Exchange On-Premises > Organization Configuration > Mailbox
- Select the Database Management tab page
- Right-mouse click a mailbox database, go to Properties
- In the Mailbox Database Properties go to the tab page Client Settings
- If the Offline address book is empty, click the [Browse...] button and select your OAB.
- Repeat step 5 – 7 for EACH mailbox database
When this is done, restart your Outlook 2010 application on the client.
You should now be able to download the OAB again via File > Account Settings > Download Address Book…
If this didn’t work for you, please comment how you fixed it.