Thanks, worked fine.
This also fixed another issue: In a cluster of two machines I was not able to migrate a VM to the other machine's ZVF storage. After the transfer has been completed, proxmox would complain that it had a stale disk images on the new machine in both of the ZFS storages and the migration would fail.
Maybe the GUI should prevent creating multiple ZFS storages on the same zpool as this leads to problems.
....Volker