Solving Outlook Address Book Download Error

Introduction
As there is a change in Exchange 2013 and above versions, Address book distribution and offline address are stored in organization mailbox.

CAS server proxy the request to nearest/available database mounted mailbox server which contains system/Arbitration mailbox.

Step1.  Autodiscover
Make sure Autodiscover is correct using the following command.

Get-clientAccessServer | fl Name,AutoDiscoverServiceInternalUri

If it is not set or set to unresolvable name please set it using the following command.

Set-ClientAccessServer -Identity server1 -AutoDiscoverServiceInternalUri "https://mail.emaildomain.com/autodiscover/autodiscover.xml"

Step2. OAB URL configuration.
Ensure OAB URLs are set in Exchange using the following command.

Get-OabVirtualDirectory -server SERVERNAME

Please set your URLS if its incorrectly set.

Set-OabVirtualDirectory -Identity "server1\oab (default web site)" -InternalUrl "https://mail.exchange.online/oab" -ExternalUrl "https://mail.exchange.online/oab"

You should then see the following.

In Exchange 2013 and above versions you can set from EAC.

Navigate to Servers-->Virtual directories. Click on "OAB(Default WebSite)" and Click "Edit" button.

Enter the Internal and External URLs with the same FQDN and click "save". 

Step3. OAB Tagging in database.
Make sure OAB is tagged in all the databases using the below command. 

Get-MailboxDatabase | Ft Name, *book*

If you see any database not tagged, tag the OAB to the database using the below command.

Get-Mailboxdatabase | Set-MailboxDatabase -OfflineAddressBook “Default Offline Address Book”

Step4. Ensure System/Arbitration mailboxes in place. 
Run the following command and ensure it exists.

Get-Mailbox -Arbitration | where {$_.PersistedCapabilities -like “*OAB*”} | ft Name, Servername, Database

It should appear like this:
 
If it is missing you could create a new one and tag the databases using the folowing commands.

 New-OfflineAddressBook -Name “NewOfflineAddressBook” -AddressLists “\Default Global Address List” -VirtualDirectories “SERVERNAME\OAB (Default Web Site)”

And tag the the new OAB to the database using the following command.

Get-Mailboxdatabase | Set-MailboxDatabase -OfflineAddressBook “NewOfflineAddressBook”

Step5. Creation of OAB mailbox/generation.
Create a new mailbox if the above steps didn't fix.

Create New-Mailbox -Arbitration -Name “OABGen” -UserPrincipalName OABGen@GT.KW –DisplayName “OAB Generation Mailbox” -Database DB30

Set OAB Generation

Set-Mailbox -Arbitration OABGen@GT.KW -OABGen $true

Step6. Ensure OAB database is healthy.

Get-Mailbox -Arbitration | where {$_.PersistedCapabilities -like “*OAB*”} | ft Name, Servername, Database

You will see something like this:

Step7. Restart IIS/OAB related services. 
Restart the MS Exchange File distribution service and MS Exchange System attendant service. If restarting the services does not help, please try restarting the servers as well as in some cases the restart helped.