We've just enabled Security Defaults for a tenant, and are having an issue adding the account to Mac Mail on a Macbook running MacOS Catalina.
The user previously had the account setup and working, so after we enabled security defaults we removed the account and went to re-add it. Everything appears to start off fine; the setup prompts to Sign in, it loads the Microsoft/365 sign-in popup, the Authenticator app receives the approval notification, but when you tap approve the form on the computer changes to a loading spinner and doesn't go any further.
We've tested this same user account on a different Macbook with the same OS version and the setup works and completes perfectly. We've completed the standard troubleshooting of rebooting, changing WIFI networks, checking for any security software, VPN's or security appliances (none present).
Moving to Outlook is not an option unfortunately, neither is switching from security defaults to Conditional Access (no Azure AD license).
Has anyone encountered this before, or can suggest what we can look to next to troubleshoot?
