Status 401 unauthorized means there is an Access issue to the source model. I would double check the following:
1. Are you logged in as Single Sign On (SSO) user or through the Anaplan online login portal? If you are logged in as a SSO and the source model requires you to login through the online portal you will receive this error.
2. Double check and make sure your access for the model wasn't modified.
3. Your session might have expired try reauthenticating
Let me know if these suggestions help, If not I would suggest contacting firstname.lastname@example.org and provide details with your source and target model import details and we can further look at your specific case.