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Hi there, I have this problem of faulty backup on NFS share, here is the mail log:

XSIBACKUP-FREE 9.1.9 AT HOST:
VMware ESXi 6.0.0 build-6765062
Mirroring following VMs to /vmfs/volumes/NAS
VM Name     State     Size     Stop     Copy     Start     Time (min)     Speed (mb/s)
WIN-DC    ON    22G    NO (hot backup)    KO!    -     0     1238
Last notice raised for the above VM:
[ Fri Nov 24 00:01:03 UTC 2017 ] ERROR RBAKSRV1 details: Unable to determine the type of server parsed   
WIN-DATA    ON    17G    NO (hot backup)    KO!    -     0     953
Last notice raised for the above VM:
[ Fri Nov 24 00:01:03 UTC 2017 ] ERROR RBAKSRV1 details: Unable to determine the type of server parsed   

Path /vmfs/volumes/NAS is an NFS share on an old Synology NAS with async, no root squash...all default settings.
Both two VMs are WINDOWS 2016 SERVER.
I haven't tried with a different OS VMs e.g. Linux because there aren't on this host.
Of course the NFS destination is writable from VMware host, being tested from shell and from vSphere client as well.
The OS version of Synology is very old, a DSM 3.1.
Any idea would be very appreciated, thanks

E.