Symptoms
In a Microsoft Exchange Server 2016 environment, you use Outlook Web Access (OWA) to create a new category. If the name of the new category is the same as an existing one but has a different case sensitivity, the new category is not created.
Cause
This issue occurs because OWA does not validate case-sensitive for category.
Resolution
To resolve this issue, install Cumulative Update 5 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.