While migrating from Zimbra you noticed following error in your logs:
“Failed to authenticate:permission denied: need right: adminLoginAs for account [user’s account]”.
The admin account configured in your source connector doesn’t have enough permissions to access user’s account. It should be granted LoginAsAdmin rights in order to have access to users mailboxes.
Please assign necessary rights to your admin account and restart the migration.
You can follow up this article https://www.zimbra.com/docs/ne/6.0.6/administration_guide/delegated_admin.10.8.html.