An error occurred when you tried to access your mailbox because a server with information about you and your mailbox couldnt be found.
User host address: ........
OWA version: .........
Exception type: Microsoft.Exchange.Data.Directory.Recipient.NonUniqueRecipientException
Exception message: Multiple objects with Sid S-1-5-XXXXXXXXXXXXXXXXXXXXXXXXXXX were found
The problem here is that the msExchangeMasterAccountSid value has been set one more than one object.
Run these commands to export the list of SID's and MsExchMasterAccountSid to quickly find out the conflicting object(s)
Get-ADUser -SearchBase "OU=Users,DC=heerwegh,DC=com" -filter * -Properties objectsid,msexchmasteraccountsid | Select alias,msexchmasteraccountsid,objectsid | Out-GridView