Recently trying to setup a user to be able to access IRM documents out of a SharePoint library , and when opening the documents from the browser , an error would appear

access denied contact your administrator

I tested and the user could protect documents themselves showing the IRM platform was working. I tested setting up a test AD user and getting them to sign into that and still would open in login in a ‘Modern Authentication” login loop.

In the end a fix was to download the file before opening it ( Instead of clicking from the browser )