If you are on Dynamics 365 for Operations 7.1 and planning to uptake the latest set of binary hotfixes, expect to see the above error when trying to activate MPOS.

Sergey from Microsoft has described the change that is triggered by binary hotfix KB4051347 in below link in great detail. Just one note though if you are on 7.1 you will not find the event ID 40619 in the event viewer. Its only available 7.2 onwards.

https://community.dynamics.com/ax/b/axforretail/archive/2017/11/06/mpos-aad-changes-in-monthly-update-4

In case you are interested to know more about these changes the below link is a great video that describes the differences between ADAL versus MSAL –https://channel9.msdn.com/Events/Build/2017/P4031

-Hitesh Manglani