Consider the following scenario:
You have an Exchange Server 2013 or Exchange Server 2016 environment.
You have multiple server sites.
The client access server (CAS) role is installed on all server sites.
You have cross-site Database Availability Group (DAG) configured on each server site.
You have forms-based authentication disabled and windows authentication enabled in each Exchange Control Panel (ECP) virtual directory on all CAS roles.
You use the same configuration for the ExternalURL value for ECP for all CAS roles.
In this scenario, monitoring probes of ECP.Proxy health checks fail on all CAS roles. This issue occurs when the CAS site is different from the site that the health mailbox database is mounted on.
This issue occurs because unexpected redirections are generated for monitoring probes.
Exchange Server 2016
To resolve this issue, install Cumulative Update 9 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Exchange Server 2013
To resolve this issue, install Cumulative Update 20 for Exchange Server 2013 ora later cumulative update for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.