Error: “Failed to access public folders. An internal server error occurred”

KB ID: 4033
Product: Veeam Backup for Microsoft 365 | Veeam Backup for Microsoft Office 365 2.0 | Veeam Backup for Microsoft Office 365 3.0 | Veeam Backup for Microsoft Office 365 4.0
Published: 2020-10-20
Last Modified: 2021-04-09
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Challenge

NOTE:
The challenge and solution described in this article apply to Office 365 organizations using basic authentication or modern authentication with allowed legacy protocols. For Office 365 organizations that use modern app-only authentication, Public Folder mailboxes backup is not supported. For more information, see KB3146.

Public folder mailbox backup fails with the following error:

Processing mailbox Mailbox1_########@.onmicrosoft.com failed with error: Failed to access public folders.. An internal server error occurred. The operation failed., Mailbox must be accessed as owner. Owner: ####; Accessing user: /o=ExchangeLabs/ou=Exchange Administrative Group (####)/cn=Recipients/cn=-Mailbox1

Cause

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.

Solution

The solution is to disable impersonation for public folder mailbox backup and provide the Veeam service account with direct access to the mailbox.

Before you apply the solution, make sure that two prerequisites are met:

  1. Veeam service account has an Exchange Online license and an active mailbox within the organization.
  2. Veeam service account has the Owner role to access a Public Folders mailbox and all of its subfolders.

To disable impersonation, do the following:

  1. Open the Veeam Backup for Microsoft 365 console and stop active backup jobs (if any).
  2. Open the Services.msc console on a proxy server that is responsible for processing public folders.
    To find out which proxy server is used, in the Veeam Backup for Microsoft 365 console, right-click a backup job, select Edit and go to the Specify Backup Proxy and Repository step of the wizard.
  3.  In the Services.msc console, stop the Veeam Backup Proxy for Microsoft Office 365 Service.
  4. Create a backup copy of the C:\ProgramData\Veeam\Backup365\Proxy.xml file by copying it to another location.
  5. Run Notepad as Administrator and click File > Open.
  6. Navigate to the C:\ProgramData\Veeam\Backup365 folder and change the view from Text Documents (*.txt) to All Files (*.*) .
  7. Open the original Proxy.xml file and add the <Source EwsImpersonatePublics="False" /> line below the <Archiver> line (As shown below). If the <Source/> line is already present and has some parameters added, add only the EwsImpersonatePublics="False" parameter there. 
Example:
<Veeam>
    <Archiver>
        <Source EwsImpersonatePublics="False" />
       .....
       .....
     </Archiver>
</Veeam>
  1. Save the Proxy.xml file.
  2. Start the Veeam Backup Proxy for Microsoft Office 365 Service.
  3. Open the Veeam Backup for Microsoft 365 console and run backup jobs with public folders.
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