Server Error: [AxiosError: Request failed with status code 400] appears when there is an authentication issue when attempting to run a Microsoft 365 sync.

Microsoft have started to enforce the use of MFA which may explain the problem as setting up MFA or changing its configuration can invalidate refresh tokens used by Microsoft Graph API integrations like the sync.

The solution for this issue is to re-authenticate the sync by using the "Sign In with Microsoft" button under Settings > Microsoft 365 within the usecure account and selecting Application Permissions as your authentication type.