While backing up a Mailbox, the job will complete successfully but your Cirrus Backup logs say completed with a warning: Failed to find group owner account.
Please assign an owner to each of the Office 365 groups you wish to have backed up. The steps to achieve this can be found in this Microsoft article.
If you have followed the steps outlined above and the issue recurs in the next backup cycle, or you need help performing these changes, you can access online help at https://support.cirrusbackup.com or reach out to our Support team via firstname.lastname@example.org.
If you are unsure of how to check your backup session logs, please see the following: View Backup Logs.