When attempting to perform a Guest Files restore from Veeam Backup Enterprise Manager, the restore fails with the error:

Cannot find the object on the mount host. Read KB4722 for more details.

This error occurs when the Guest OS partition that is being mounted is formatted with a filesystem or filesystem version that the mount Helper Host server cannot read.

For example, an Alma 9.4 Linux machine uses XFS driver version 6, but the Helper Host, which is used to mount the disks, runs Ubuntu 20.04.04 and only has XFS driver version 5. The version 5 XFS driver is incompatible with version 6, so the filesystem cannot be mounted.

As documented in the Preparing for File Search and Restore (non-Windows machines) section of the Veeam Backup Enterprise Manager user guide:

When you start guest OS file restore from Enterprise Manager, the helper host (or appliance) settings are obtained from the configuration database of the backup server. If no helper host or helper appliance configuration is found for the user account, Veeam Backup & Replication uses the configuration that was last selected at the Helper host step of the Guest File Restore wizard. Thus, before you start file-level restore from Enterprise Manager, make sure the settings are properly configured on the backup server. For details, see the Specify Helper Host section of the Veeam Backup & Replication User Guide.

Ensure that the Helper Host specified in Veeam Backup & Replication is fully compatible with the disks of the Linux machine that will be restored.