btrbk + samba shadow_copy2 + windows clients

Hello together,

I just startet a project containing a samba server on a btrfs filesystem. Because of the windows-users the server must be a shadow-copy system supported by Windows. The btrbk script is working fine and creates snapshots and backups, but the Windows clients can't see the shadow-copies.

Current configuration:
subvolume in /samba/dateiserver/filespace - with the data for samba
subvolume in /samba/dateiserver/snapshots - with the snapshots

in btrbk.conf I am using:
timestamp_format long-iso

btrbk is saving the snapshots in the directory in the following format (e.g.):
filespace.20180710T090515+0200

I tried the Help from another Issue in smb.conf:
vfs objects=shadow_copy2
shadow:snapdir=/samba/dateiserver/snapshots
shadow:format=filespace.%Y%m%dT%H%M%S%z

but I can't see any shadow copies with Windows clients.

I would be very happy about any help.
Thanks a lot
Klaus