Cannot enable UM for users

You cannot enable UM (Unified messaging) feature for users and get following errors. Enable-UMMailbox -Identity <> -UMMailboxPolicy <> -Extensions <> -PinExpired:$true -SipResourceIdentifier <> Exception: Mailbox <> couldn’t be initialized for Unified Messaging: There was an error accessing the mailbox using EWS. The requested operation failed with ResponseCode “ErrorInternalServerError”. Details : An internal server error occurred….