From not the community crew:
It might be the deprecation of basic authentication.
https://support.microsoft.com/en-us/office/modern-authentication-methods-now-needed-to-continue-syncing-outlook-email-in-non-microsoft-email-apps-c5d65390-9676-4763-b41f-d7986499a90d
Until September 16th, users signing into Outlook.com through Basic Authentication may experience recurring password prompts in Outlook and other third-party email applications. This is a known issue. After September 16th, users attempting to connect their Microsoft accounts through Basic Authentication will fail to do so.
https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/deprecation-of-basic-authentication-exchange-online
The deprecation of basic authentication also prevents the use of app passwords with apps that don't support two-step verification.
We removed the ability to use Basic authentication in Exchange Online for Exchange ActiveSync (EAS), POP, IMAP, Remote PowerShell, Exchange Web Services (EWS), Offline Address Book (OAB), Autodiscover, Outlook for Windows, and Outlook for Mac.
We also disabled SMTP AUTH in all tenants where it wasn't being used.
There is no plan for Outlook clients to support OAuth for POP and IMAP, but Outlook can connect use MAPI/HTTP (Windows clients) and EWS (Outlook for Mac).
Outlook 2007 or Outlook 2010 cannot use Modern authentication, and will eventually be unable to connect. Outlook 2013 requires a setting to enable Modern authentication, but once you configure the setting, Outlook 2013 can use Modern authentication with no issues. As announced earlier here, Outlook 2013 requires a minimum update level to connect to Exchange Online. See: New minimum Outlook for Windows version requirements for Microsoft 365.