Cannot open Mailbox as Additional Mailbox

After moving a mailbox from Exchange 2003 to Exchange 2010 a user reported that he was unable to open his mailbox.

I logged with an account with Full Mailbox permissions  and added this user’s mailbox as an additional mailbox in Outlook:

 

 

But when I tried to expand the Mailbox in Outlook the Hourglass cursor appeared and after some time I got an error message:

 

Strange enough I was able to access the Mailbox with Outlook Profile configured with the mailbox as the primary account.

After some troubleshooting I noticed there was a conflict with the legacyExchangeDN.

A small PowerShell script confirmed this:

$Mbx = Get-Mailbox "John Doe"
Write-Host "Searching for: " $Mbx.LegacyExchangeDN
Get-Mailbox $Mbx.LegacyExchangeDN

The script should return one mailbox but in my case it returned two:

 

The 2nd mailbox had the legacyExchangeDN added as X500 address:

To resolve the conflict we need to adjust either the legacyExchangeDN from this mailbox or delete the X500 address from the other mailbox. Deleting the X500 address seemed the safest approach so that’s what I did.

I retested this scenario with Outlook 2010 and the results were the same but additionally an error was logged in the Evenlog on one of the CAS servers: