Source: http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/A_5064-9-Steps-to-end-OAB-nightmares.html

I spent quite a bit of time testing this procedure in an Exchange 2007 SP3 on Windows 2008 lab (not SBS). So it should work for Exchange 2010 too. Take care when SBS or multiple CAS servers are involved. I have tried to include additional information for those 2 scenarios.

This procedure will help reset your OAB Virtual Directory back to it’s default settings and permissions. It should also cause the Web Distribution of the OAB to start working, so then Outlook 2007/2010 clients can download it properly. If your public folders are available and you are doing PF distribution of the OAB then this procedure should cause the PF version of the OAB to be generated too.

Please try and be on the latest Service Packs for Windows and Exchange as this will solve lots of issues for you and is better for supportability.

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OAB Folder & Virtual Directory reset: 

1

On the CAS server/s stop the Microsoft Exchange File Distribution Service. You can do this from the command prompt with ‘net stop MSExchangeFDS‘

2

Untick ‘Enable Web-based Distribution’ of OAB: From EMC–>Org–>Mailbox–>Offline Address Book–>double click on your Offline Address Book (normally called ‘Default Offline Address Book’)–>’Distribution’ Tab–>Untick ‘Enable Web-based Distribution’ then press Apply. You can also do it from EMS with the command ‘Get-OfflineAddressBook | Set-OfflineAddressBook -VirtualDirectories:$Null’. This should of resulted in the <Drive>:\Program Files\Microsoft\Exchange Server\ClientAccess\OAB\<GUID> folder being removed from your CAS server/s

3

Open EMS and do “Get-OABVirtualDirectory | Remove-OABVirtualDirectory -confirm:$false”. This will remove the OAB Virtual Directory from all of your CAS server/s. So use the -WebSiteName parameter if you have more than 1 CAS server and you only want to perform this reset on 1 server.

4

Delete/Rename the <Drive>:\Program Files\Microsoft\Exchange Server\ClientAccess\OAB folder on the CAS server/s

5

Delete/rename the <Drive>:\Program Files\Microsoft\Exchange Server\ExchangeOAB folder on the OAB Generating server (usually a mailbox server).

6

Open EMS and do “New-OABVirtualDirectory -Server <CASServerName>”. This will create the OAB Virtual Directory on the CAS server and will create the  <Drive>:\Program Files\Microsoft\Exchange Server\ClientAccess\OAB folder on the CAS server too. Repeat this step for each of your CAS servers. If you only have one CAS server then you do not need theServer parameter. This will also set the InternalURL value of the OABVirtualDirectory to “http://<CASfqdn>/OAB” which is fine if you don’t want to use SSL internally to download the OAB (this is the default Exchange setting). WARNING: SBS seems to default to HTTPs on the OAB VD because of the Bindings on the SBS Web Applications web site, so I suggest you use HTTPs, or if you really want to use HTTP then look at the Bindings and which HostNames will allow you to connect to the SBS Web Applications web site on port 80. on SBS use ‘New-OABVirtualDirectory -WebSiteName “SBS  Web Applications”‘ To ensure the OAB VD is created under the correct web site.

7

Open EMS and do ‘Get-OfflineAddressBook | Set-OfflineAddressBook -VirtualDirectories “<CASServerName>\oab (default web site)”‘. On a SBS server the (default web site) is normally (SBS Web Applications), so open IIS and confirm what the web site name is that contains your Exchange Virtual Directories and then use that value instead of (default web site) if you need to. You can do this step in EMC as you may find it easier, especially if you are running multiple CAS servers.

8

Open EMS and do “Get-OfflineAddressBook | Update-OfflineAddressBook“. This will recreate and populate the<Drive>:\Program Files\Microsoft\Exchange Server\ExchangeOAB folder on the OAB Generating server (usually a mailbox server) and it will also share the folder out as ExchangeOAB.

9

On the CAS server/s start the Microsoft Exchange File Distribution Service. You can do this from the command prompt with “net start MSExchangeFDS“. This will copy the <Drive>:\Program Files\Microsoft\Exchange Server\ExchangeOAB\<GUID> folder on the OAB Generating server to the  <Drive>:\Program Files\Microsoft\Exchange Server\ClientAccess\OAB folder on the CAS server/s.
Recycle the MSExchangeAutodiscoverAppPool Application Pool in IIS, so autodiscover picks up the new OAB URLs. Then restart Outlook so it reads the new Autodiscover information about the OAB URLs and test Downloading the Offline Address Book from an internal Outlook 2007/2010 client and it should now work.