Basic auth deprecation is coming in October. The service has been temporarily disabling basic auth so that tenant admins become aware they are using it. If you log into the M365 portal with your tenant admin account you will likely find messages in the message centre about this e.g. https://admin.microsoft.com/Adminportal/Home?source=applauncher#/MessageCenter.
Outlook Desktop does not support POP or IMAP in the service with modern auth so those won’t work after October. The best thing to do is to switch your account to an Exchange connection. Here is more about the basic auth deprecation, Deprecation of Basic authentication in Exchange Online | Microsoft Docs.