Occasionally, when a presentation is opened while a modal (add-in) dialog is open, PowerPoint seems to get into an infinite loop reading the following registry value:
HKEYCURRENTUSER\Software\Microsoft\Office\16.0\Common\Identity\DisableADALatopWAMOverride

PowerPoint does not show as "Not responding" but effectively it is not responding to any clicks.

We have only seen this behavior on machines where DisableADALatopWAMOverride was set to 1. This cannot reliably be reproduced but happens on different machines for different users. All of this seems to be related to Office 365 authentication.