This is related to SharePoint 2019 on-premise, interfacing with ADFS and Office Online Server (formally known as Office Web Apps).
I have a single-server SharePoint Farm with multiple web applications. The base SharePoint Web Application protected using Kerberos + Windows Authentication. When I open documents in the SharePoint Document Libraries, they open in Office Online Server (OOS).
The base web application is also extended, and protected using ADFS for authentication. When I attempt to open the same documents from that web application, I am taken to the following screen on the Office Web Apps server:
For the past 10 days I have been trying to work with Microsoft Support on the issue, but they are useless. I've been bounced between the SharePoint and Office Web Apps teams so many times that I'm beyond frustrated.
Anyone with prior experience that is willing to help would be much appreciated. Everything appears to work fine when accessing SharePoint through the Kerberos-protected web application. The issues only start after attempting to access through the ADFS-protected web application.