“Object reference not set to an instance of an object” error and Setup fails in Exchange 2019 CU10 and 2016 CU21

Symptoms

When you try to set up Microsoft Exchange Server 2019 Cumulative Update 10 (CU10) or Microsoft Exchange Server 2016 Cumulative Update 21 (CU21), Setup fails at the "PrepareAD" step and returns the following error message: 

System.NullReferenceException: Object reference not set to an instance of an object.

Cause

Setup fails if it cannot find or access the CN=Computers.<domain>.<com> container.

Workaround

If the Computers container was renamed to some other name, use Active Directory Users and Computers to revert the container name to "Computers."

If the Computers container was deleted or you cannot rename the original Computers container, use the following steps to create a dummy Computers container to enable Setup to continue:

  1. Open ADSI Edit (Adsiedit.msc).

  2. Right-click ADSI Edit, and select Connect to.

    Screenshot that shows the steps to select "ADSI Edit" and "Connect to."

  3. In the Connection Settings window in the Name field, make sure that Default naming context is selected on the drop-down list, and then select OK.

    Screenshot that shows the Connection Settings dialog box.

  4. Expand Default naming context [<domain>], right-click DC=<domain>,DC=com, and then select New > Object.

  5. In the Create Object window in the class list, select container, and then select Next.

    Screenshot that shows selecting a class in the Create Object window.

  6. In the Value field, enter Computers, and then select Next.

    Screenshot that shows adding a value in the Create Object window.

  7. Select Finish.

    Screenshot that shows selecting "Finish" in the Create Object window.

  8. Force Active Directory (AD) replication.

  9. Resume or start Exchange Setup again.

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