When you run the Setup program to install Microsoft Exchange Server 2016 in build-to-build upgrade mode, the Exchange ActiveSync (EAS) web.config file on the Client Access Front End (CAFE) server is not updated.
This issue occurs because there is no existing method to make necessary changes to update the web.config file for build-to-build upgrades.
To resolve this problem, install Cumulative Update 6 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
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.