I Recently upgraded my Exchange Organization with some Exchange 2010 server.
Obviously you start with the CAS Server 2010. Opening the EMC returned a strange error : An IIS directory entry couldn’t be created. The error message is Access is denied. HResult = –2147024891
That was strange as the setup did not return any errors

After some searching on the internet I found the answer on this site:
http://msexchangegeek.com/2009/09/18/get-owavirtualdirectory-returns-an-iis-directory-entry-couldnt-be-created-the-error-message-is-access-is-denied/

Strangely enough this error is spawned because there is/are Exchange 2007 CAS Server(s) that do not give permissions to Exchange 2010 to enumerate IIS.
As you may know is this very odd because the EMC 2010 does not display any Exchange 2007 Server!
The Solution is to add the security group “Exchange Trusted Subsystem” as member of Local Administrators group on an ALL Exchange Server 2007 boxes

In my opinion this should be solved by the Exchange 2010 setup process or some additional ADPREP parameter that takes care of this.