Veeam ONE generates “Failed to open VM attachment” alarm.
Veeam ONE triggers “Failed to open VM attachment” alarm when the following Windows events are registered on the Hyper-V host:
These events are logged when the Hyper-V host or the Cluster node cannot access some of the VM virtual disks, configuration files, virtual memory, saving state or snapshot files.
NOTE: In most cases, this happens when the files are located on the shared storages (for example CSV or SMB shares).
Make sure that the mentioned in the alarm details VM file is accessible from the Hyper-V host or the Hyper-V Cluster where VM is residing.
If the file is accessible, but the events continue to be triggered in the Windows Event log, you need to contact Microsoft support for further investigation.
NOTE: For SMB shares you need to additionally configure SMB delegation on the Hyper-V side.
To enable SMB delegation, refer to the Microsoft documentation: