Assume that Microsoft Exchange Server 2019 or Exchange Server 2016 is running on Windows Server 2016. When users use an invalid Windows language, such as English (Switzerland), as the primary language of their operating system, the Outlook on the Web client cannot be loaded. Additionally, Events 1003 and 4999 may be reported in Event Viewer.
This issue occurs because Exchange Server uses the Windows Language Code Identifier (LCID) to identify the language, and the LCID of English (Switzerland) isn't unique.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
To fix this issue, install one of the following updates:
For Exchange Server 2019, install Cumulative Update 1 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.
For Exchange Server 2016, install Cumulative Update 12 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Learn about the terminology that Microsoft uses to describe software updates.