We might get this below error on installing the first Exchange 2016 on a coexistence setup with Exchange 2013 or Exchange 2010.
When looking through the setup logs we can find the below reason to stop the installation.
Write-ExchangeSetupLog -Warning (“Tried to create new default OAB but the object already exists; it may have been created by another instance of setup.”)
Open ADSI Edit, go to CN=Configuration,DC=domainname,DC=local\CN=Services\CN=Microsoft Exchange\CN=Container\CN=Address Lists Container\CN=Offline Address Lists
Right click on the Exchange 2010/2013 OAB (according to the legacy exchange version you have )and click Properties.
Look for the value ‘msExchOABDefault‘ and Make this value to Not Set or False and then click apply ok.
What is this msExchOABDefault ?
This is a Boolean attribute in the offline address book properties.
The already existing Exchange setup might be having this value set to True.
This value can be either True ,false or Not Set .
If its set to true then this will be the offline address book for any mailbox store, databases in the organization.
Why it fails with this value True is because the Exchange 2016 setup successfully creates the new OAB container in the ADSI EDIT during the installation.When it attempts to set this value to True it fails because the old one has already value set to True.
There can be only one Offline Address Book in a Organization which value can be set to True which is the default OAB.
Now rerun the setup and it should be completed without any issues
After successful installation we can see the default value set to True on the higher version of Exchange as below
Be careful while performing the steps on the ADSI EDIT container since deleting any objects accidentally will lead to a big issue. Better to take a backup before performing any actions on the ADSI Edit.
MVP – Office Servers & Services