The “Failed to find Team mailbox owner account” error can be caused by the owner of the mailbox being moved, or if the owner no longer has a license assigned. Resolving either of these will also resolve your M365 issues. Here are the steps to resolving the cause of either issue:
If the owner was removed from the mailbox:
- Login to the Teams admin center.
- Expand Teams, then select to manage teams, and select the Team that is causing the warning.
- Select the Members tab to either re-add and re-assign the original owner, or select a different member as the owner.
Assigned owner does not have a license:
- Navigate to your M365 admin center.
- Open Users, and find the owner of the mailbox.
- Under the Assign product license tab, assign a valid license.
As these are two of the most common causes of the “Failed to find Team mailbox owner account” error, there may be other issues causing the error.
Source: https://www.managecast.com/veeam-backup-for-microsoft-365-failed-to-find-team-mailbox-owner-account/