Today we are announcing the availability of quarterly Cumulative Updates (CUs) for Exchange Server 2016 and Exchange Server 2019. These CUs include fixes for customer reported issues, all previously ...
Updated Jun 29, 2021
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Blog Post
Finally, a solution to this stupid problem (crash during ADPREP).
I had a RODC (read-only domain controller) in a sub-OU under Domain Controllers. Moved it to the root Domain Controllers folder, ran the upgrade again, and it is finally successful (well, another crash regarding C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\SharedWebConfig.config not existing, fixed https://social.technet.microsoft.com/Forums/en-US/a1c5c0f6-73db-4845-98b8-7a3c3031d3ed/exchange-2013-cu15-cas-upgrade-fails?forum=exchangesvrdeploy then ran it again and it worked).
Now I will move the RODC back to where it was, because of group policies that apply to that OU which are different than the root DC folder.
Thanks, dvolkov83 and others!