Symptoms
Consider the following scenario:
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A user connects to the Exchange Admin Center (https://<ExchangeCAS>/ECP) on a Microsoft Exchange Server 2013 Client Access Server (CAS) with Forms-Based Authentication disabled.
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The CAS is in a different site than the user's mailbox.
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A CAS in the site of the user's mailbox has an ExternalUrl value set.
In this scenario, instead of being silently redirected, the user receives the following error message:
Error executing child request for /owa/auth/errorFE.aspx.
Cause
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Resolution
To fix this issue, install Cumulative Update 12 or a later cumulative update for Exchange Server 2013.
Workaround
Microsoft is currently investigating this issue.
To temporarily work around this issue, copy the version folder from C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\owa\auth to C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\ecp\auth.
For example, if the CAS that the user connects to is running Exchange Server 2013 Cumulative Update 9, copy the 15.0.1104 folder from C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\owa\auth to C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\ecp\auth. When you do this, the final folder structure is defined as follows:
C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\ecp\auth\15.0.1104
Note Any updates to Exchange will require that the new version folder also be copied.
For Exchange Server updates and their build numbers, see Exchange Server Updates: build numbers and release dates.
More Information
This issue applies only to users who have Exchange Server 2013 mailboxes that connect to CAS without Forms-Based Authentication and who need a redirect cross-site. There's a separate issue related to users without mailboxes or with legacy mailboxes who receive the same error message. In that scenario, ECP requires the org mailbox, and adding ?ExchClientVer=15 is a simple workaround.