Mounting snapshots with SDCLI on Windows does not work

I'm able to create/rename a snapshot but get an error when I try to connect the snapshot to a local drive letter:

C:\Windows\system32>sdcli snap rename -d D -o hellmann1 -n hellmann
The operation completed successfully.

C:\Windows\system32>sdcli snap mount -k D -s hellmann -d X
Initializing...DEBLNCM204 : Checking input parameters
Unable to connect to the LUN in Snapshot. Check event logs for more details.

In the event viewer I see:

Failed to connect the LUN in Snapshot copy (hellmann) as drive X on the computer (DEBLNCM204). 

LUN Name = viewstore1.lun
Storage Path = /vol/viewstore1ad/
Protocol Type = HTTPS
Storage System Name = iscsi

Error code : ZAPI: An attempt to get snapmirror destinations list for the 'iscsi:viewstore1ad' source failed on the storage system iscsi. Error code: -1. Error description: Can't connect to host (err=-11001)..

It worked for years with our old filer (8.2.4P4 7-Mode).

With a LUN connected to our new filer (9.1P1) it does not work.

Any ideas?

Thanks,

Michael