I have two o365 accounts from different companies. One of them works fine the other I get "user authenticated but not connected" I turned off ipv6 globally and specifically for outlook.office.com and tried relogging even revoked the app and allowed it again. Sending emails somehow works but it keeps loading even after the email is received. I'm on Linux tried both native and flatpak app. These account also work just fine through davmail. The irony is that the account which doesn't work recommends Linux users to use thunderbird that's what promoted me to switch to it instead of davmail.