Microsoft has deprecated Basic Authentication (login / password) and has turned it off for most clients, switching to MFA instead (Multi-factor authentication).

Please see this for more information:

https://portal.office.com/adminportal/home#/servicehealth/:/alerts/EX444315

For this reason, the normal login / password authentication flow won't work. Any Exchange account added to BusyCal and BusyContacts will continue to fail.

You should delete this account from the app and then re-add as a "Office 365" account instead of "Exchange" for it to make an attempt to refresh account settings and re-sync using Modern Authentication (MFA). Here's how:

• Open the app's Preferences

• Click on the "Accounts" tab at the top, and then select the account in the left sidebar. Click the minus [-] button to remove it (visible at the bottom of the sidebar).

• Then re-add the account by clicking the plus [+] button, selecting the "Office 365" account type, and entering your account info.

Microsoft has also advised that clients can re-enable basic authentication, if required, by following this process:

https://techcommunity.microsoft.com/t5/exchange-team-blog/basic-authentication-deprecation-in-exchange-online-september/ba-p/3609437

NOTE: In case the app refuses to connect to your Office 365 account after following the steps above, and you see a "Approval required by admin" message (or similar) when trying to login - you will need to contact your Office 365 admin for an explicit approval. They will need to mark BusyCal / BusyContacts as a "Trusted" application. Some organizations may choose to block all 3rd party apps for security reasons and may only choose to trust specific applications when requested. We're happy to assist with the approval process if the admins would be willing to approve our apps.

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