Symptoms
In a Microsoft Exchange Server 2016 environment, assume that the Active Directory forest root domain is configured as a single label domain that has one or two characters, such as ab. When a user or administrator tries to access a website (for example, Outlook Web App (OWA), Exchange Control Panel (ECP), Autodiscover and so on) through the Client Access services (CAS), an unhandled exception occurs.
Cause
This issue occurs because the single label domain isn't correctly handled in forest name validation.
Resolution
To fix this issue, install Cumulative Update 9 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
References
Learn about the terminology that Microsoft uses to describe software updates.
