If when running Veeam backups you receive an error processing the public folders like this:

Additional Details: 10/14/2020 12:22:09 PM :: Processing mailbox CSD-Primary_7337b59e@csd7.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: 529bc00e-6ad9-42ff-bf95-07d270eb30a6; Accessing user: /o=ExchangeLabs/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=645fbc49969645e2a447e31331c8abc9-CSD-Primary :: 0:00:51

Response from Veeam:

1) We've been seeing instances of this recently with Public folders. The fix here is to ensure that the service account VBO is using is licensed and to set it as owner of the Public folders in O365. This is usually happening for public folders missing a "default" user.

2) In addition to being the owner or reviewer of the public folder, we'll also need to disable ews impersonation. How to do this is detailed in the very last section of this KB at the bottom: https://www.veeam.com/kb3093

3) You don't need to moving anything. Simply disable impersonation by following the instructions at the end of that article. https://www.veeam.com/kb3093

-details-To disable impersonation, do the following:

1.  Open the Veeam Backup for Microsoft Office 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 Office 365 console, right-click a backup job, select Edit and go to the Specify Backup Proxy and Repository step.

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.  Open the original Proxy.xml file using any text editor and add the <Source EwsImpersonatePublics="False" /> line between <Veeam><Archiver> tags.  Example:<Veeam>    <Archiver>        <Source EwsImpersonatePublics="False" />       .....       .....     </Archiver></Veeam>

6.  Save the Proxy.xml file.7.  Start the Veeam Backup Proxy for Microsoft Office 365 Service.8.  Open the Veeam Backup for Microsoft Office 365 console and run backup jobs with public folders.

That's it.