We are also stuck on the same issue. Try if this works for you. I have an instance working on V13 where this is working, but not on V14.
Office 365 settings - Frappe Framework - Frappe Forum
I think it is mandatory in V14 to convert to Oauth2.0 which works through open api app but I have not been able to crack the correct steps to make it working. If you come across a solution please inform. Thanks