You can't access a mailbox by using Exchange Web Services (EWS) after the mailbox is moved to Microsoft Exchange Online in Microsoft Office 365 Dedicated/ITAR (vNext). Additionally, you receive error messages that resemble the following:
An internal server error occurred. The operation failed.
This issue occurs because account credentials aren't accepted in the current format.
To resolve this issue, make sure that the user principal name (UPN) of the account's on-premises Active Directory schema is being used during the authentication process. This UPN is the logon for Office 365 Dedicated/ITAR (vNext). Also, this UPN differs from on-premises Exchange or legacy Dedicated/ITAR service that enables the Domain\sAMAccountName attribute to be used.