Exchange Online email applications stopped signing in, or keep asking for passwords? Start here.

Matt_Desbiens Actually, Office versions newer than Office 2013 should not require any registry keys to make modern authentication work. We do mention registry keys in the context of newer versions of Office, though, but this is usually a result of in-place upgrades of Office versions on machines, so the presence of registry keys CAN make a difference (for example if EnableADAL key was set to 0 long ago, then even a newer version of Outlook will refuse to use modern auth). If you have verified that modern auth was enabled on the tenant level and still cannot figure this out, please open a support ticket via the "Help & Support" floating button in Microsoft 365 admin center.