Permission issue on Windows 2012 R2 when calling VSS

When calling quiesce snapshot, it fails (despite tools up to date and XS 7.1 with all patches applied). I'm sure the VSS service is up and works when calling it manually (from a Windows terminal).

The only lead I have is an error in the Windows logs at the time when I ask a quiesce snapshot. When the VSS is called, it failed because of a permission issue (??!!). Error code are "error 13" or "error 12292".

Is there anything to know to have VSS via Xen tools working regarding permissions on Windows? Both problematic VMs are running AD services.