We're in the process of migrating users from Exchange 2016 on-premise to 365. I have some migrated users reporting some strange issues after they've been migrated.
The first issue is some users are seeing what starts to be (presumably) a Microsoft Account authentication box, but then it goes away. It may be up for a microsecond, or it may be up for 2-3 seconds. It doesn't seem to occur at regular intervals, and I've never seen it myself. A user finally was able to hit Print Screen in time to capture it, though.

In addition, many users are being prompted for credentials post-migration. This may happen 3-4 times per day. The authentication box is a Windows authentication box however, not a Microsoft Account authentication box. Their password is not accepted when entered.

Sometimes it even shows Windows Mail instead of Microsoft Outlook.
Are these issues related? All machines are Windows 10 machines joined to our local domain.