Hi Torsten,
Harvest should update its instance cache every 1 hour or after restart, so normally this shouldn't happen. Can your customer run Harvest in foreground mode and check whether the output metrics contain the old or new SVM names?
Starting Harvest in foreground/verbose mode:
./netapp-worker -poller POLLER -v
Or to get only the output metrics:
./netapp-worker -poller POLLER -v | grep "M="
(Replace POLLER with the poller name).