Hey @Stuart Schifter,
I have seen this from time to time and I would not worry about it too much as long as they could log in.
How Pro Plus works is it essentially tries to login every time the application boots up. It then stores that successful login and deviceID inside of its logs both on the back end of O365, and in the application itself. Inside of the application, the authentication stays good for 30 days, at which point if it has not checked in again, it will go into a reduced functionality mode, where you can open and view documents, for instance, but not edit them.
Because this tries to re-auth with every login, I have noticed that at times, especially in larger organizations, sometimes it just wants you to re-enter in your credentials. For instance at my last job, a company of about 8000 people, I probably got the prompt every other week or so.
Let the client know its ok, and somewhat normal. You can then load up their subscription in Office 365 and verify the devices/systems he has connected to his proplus to make him feel better about the situation.
Check out the "managing active installs" in this link:
https://docs.microsoft.com/en-us/deployoffice/overview-of-licensing-and-activation-in-office-365-pro...
Adam
