Hi @RASH MAAR ,
According to the description of the security update for KB6000871. Exchange services might remain in a disable state after you install this security update. Please try to setup the start up type to Automatic and start the affected Exchange services manually.
For more infomration you could refer to: Description of the security update for Microsoft Exchange Server 2019, 2016, and 2013: March 2, 2021 (KB5000871)
According to the error infromation, the Microsoft Exchange Active Directory Topology service did not start successfully. Almost all Exchange-related services depend on the Microsoft Exchange Active Directory Topology service. Please follow the method in the link below to increase the value that caused the service timeout. Then restart the computer and try to start the Microsoft Exchange Active Directory Topology service again.
For more information: A slow service does not start due to time-out error in Windows
In addition, incorrectly modifying the registry will cause serious consequences, so please backup in advance: How to back up and restore the registry in Windows
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