Consider the following scenario:
You separately deploy Microsoft Exchange Server 2019 and Exchange Server 2016 on Windows Server 2019, Windows Server 2016 in the same organization.
You create a database availability group (DAG) and add one of the mailbox servers like Exchange 2016 mailbox server to the DAG first.
Unexpectedly, the Exchange 2019 mailbox server can also be added into the same DAG and vice versa. Two rules are violated here:
Exchange 2016 and Exchange 2019 should not be the member of same DAG.
Windows 2016 and Windows 2019 should not be the member of same DAG.
After you apply this update, the operation will fail and you will receive the following error message:
WARNING: The operation wasn't successful because an error was encountered.
This issue occurs because Exchange Server doesn't check to disallow Exchange Server 2019 and Exchange Server 2016 to exist in the same DAG.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.