Initialization Failed with Office Mail App (OMA) with Exchange 2013 CU19 and later

Problem

Launching Enterprise Vault Office Mail App (OMA) within Outlook Web Access (OWA) fails with an initialization failed error message. 

Note: Launching the Office Mail App (OMA) within Outlook works without issue.

Error Message

An HTTP network trace (Fiddler, Wireshark, built-in F12 browser tools) can be performed on the end user's workstation while generating the initialization failed error. Locate and then double-click the ExecuteEwsProxy line.

The following server response will be displayed  "The remote server returned an error: (404) Not Found."

Cause

This behavior has been observed in Exchange 2013 CU19 and later when the Exchange Mailbox Server role is separate from the Exchange Client Access Server (CAS) role. Microsoft's recommended architecture for Exchange is that the Mailbox role and CAS role be co-located.

Note: To validate the error, an 404 error will be found on the EWSProxy call in the C:\inetpub\logs\Logfile\W3SVC1 IIS log on Exchange Mailbox Server. Note that the request is sent on port 443, while the Exchange Back End website, which answers EWS calls on a server running the Mailbox role without the CAS role, is bound to port 444.

Microsoft has addressed this issue in a Knowledge Base article.

Solution

Install the CAS role on the Mailbox server. This is the preferred architecture for Exchange 2013, and it is the only architecture in subsequent versions of Exchange. Please contact Microsoft Support for further assistance.