Due to the recent changes in the Office 365 backend, public folder mailbox backup fails with the "The operation failed. Mailbox must be accessed as owner" error. This issue affects those Veeam Backup for Microsoft 365 users who back up public folder mailboxes with the default settings and have not applied the workaround provided in the KB3093.
Veeam Backup for Microsoft 365 uses impersonation to back up public folder mailboxes. The user account who accesses a public folder mailbox during backup is the Default user account. Before the recent changes in the Office 365 backend, the Default user assigned with the Owner or Reviewer roles was able to access public folder mailboxes. However, currently, even if the Owner role is assigned, the access is blocked, and the abovementioned error occurs.