A backup may be marked "Partially Successful" due to the "Instance probably doesn't exist anymore" error:
Error - (instance: i-1234567890abcdef) could not get object from aws. Instance probably doesn't exist anymore
Warning - Finished kicking off snapshots, not all started successfully.
This occurs when an instance has been terminated in AWS but not removed from the policy's Backup Targets.
In order to resolve this issue, please perform one of the following:
1) Manually remove the terminated instance from the policy's Backup Targets
2) Enable "Auto Target Removal" in the policy
When this option is enabled, the next backup will still be "partially
successful", but the terminated instance will be removed from the backup
targets, and will no longer affect this policy.