This issue started after applying RU3 for Exchange 2010 SP2.
Investigating the problem we found out that all the users that encountered this had open their mailbox stored in one DAG and at least another mailbox stored in the other DAG. This means that they were using one CAS Array to access their mailbox and another CAS Array to open the additional mailbox(s).Client Team Lead for Messaging discovered this support article from Microsoft http://support.microsoft.com/kb/2725008. It was not fully related to our issue but it contains the way to solve it.
To resolve this issue you need to disable preferred site enforcement (a new feature from Exchange 2010 SP2 RU3)To do this, create the following DWORD registry value:
Key: HKLM\System\CurrentControlSet\Services\MSExchangeRPC\ParametersSystemValue Name: EnablePreferredSiteEnforcement
Data Type: REG_DWORD
After you set the EnablePreferredSiteEnforcement value to 0, a CAS server will accept connection requests from any site and not try to redirect the request to a CAS server from the home site of the mailbox.To get more details about this new feature and how it can affect your Exchange infrastructure, you can also read this article http://info.kraftkennedy.com/blog/bid/181642/Exchange-2010-SP2-RU3-and-Cross-Site-RPC-Client-Access-Changes.