Gmail Backup v1.0 only supports password-based authentication for individual accounts. For security reasons, Google discontinued this authentication method in April 2015. This may be the cause of Gmail Backup failing to restore emails.
There are two methods to restore backup accounts:
1. Follow this link: https://www.google.com/settings/security/lesssecureapps. Select “Turn on” to provide access for less secure apps.
2. Manual account restoration:
Delete a backup account (without deleting stored emails).
Recreate the same account using Oauth2.
Select the same backup path.