I had this problem, but after I deleted the Outlook account and reinstalled it under Exchange on both the iPhone and iPad, everything has been working fine for about 2 weeks now. So fingers crossed this is the solution.
Dont hold your breath. As Others have found it’ll be back.
Without doubt a microsoft problem. When you respond to the pop up it takes you straight to the MS Live/Outlook login screen where you have to enter the password even though its still stored on your device. so clearly the microsoft
account isnt accepting the credentials the apple device is offering.
Microsoft doesnt give a damn for anyony who wont fall for their ‘everything as a service’ business model, aka pay, pay and pay again for the same old sh1te. Once a fan, i have come to despise them for their greed and arrogance. Break them up I say.
Why do you think this is a Microsoft problem. After authentication iOS receives an authentication token to log onto the system. If iOS then thinks the token is invalid, it prompts you to login again so it can retrieve a new token, which is again valid for
a few days.
A friend of mine still has an iPhone 6, which is still running iOS 12. He is not experiencing any issue at all. This makes me believe the issue lies with iOS 14 and not with Microsoft.