I have a FreeNAS iSCSI target for my ESX home lab and for the past 6 months there have been no issues.   FreeNAS has a connection to vcenter, snapshots are created and deleted without issue, at least until a day or so ago.

The following appears in the log files:

2020-05-02T02:39:26.136Z| vmx| I125: SNAPSHOT: SnapshotDiskTreeAddFromSnapshot: Trying to add snapshot Zoneminder-Snapshot795.vmsn to disk /vmfs/volumes/5c16a659-4a9c6db4-1f67-0026552a4480/Zoneminder/Zoneminder.vmdk which already has snapshot Zoneminder-Snapshot794.vmsn.

2020-05-02T02:39:26.136Z| vmx| I125: SNAPSHOT: SnapshotGenerateDeleteDisks Failed to fetch disk tree: One of the disks in this virtual machine is already in use by a virtual machine or by a snapshot (21)

2020-05-02T02:39:26.136Z| vmx| I125: SNAPSHOT: SnapshotDeleteNode failed: One of the disks in this virtual machine is already in use by a virtual machine or by a snapshot (21)

2020-05-02T02:39:26.136Z| vmx| I125: SNAPSHOT: Snapshot_Delete failed: One of the disks in this virtual machine is already in use by a virtual machine or by a snapshot (21)

2020-05-02T02:39:26.136Z| vmx| I125: VigorTransport_ServerSendResponse opID=58ae3542-46d2-4edc-9737-bc6879965ac0-178647-h5c:70019940-2d-b5-37c2 seq=15840737: Completed Snapshot request.

The message:  Trying to add snapshot to disk which already has snapshot...  Does this mean that the snapshot has already been consolidated and the snapshot file hasn't been removed correctly?  I can create new snapshots, but cannot delete them.

I've looked through the vmsd file and matched all the snapshot names, everything seems to line up.  Now I'm stuck with two VM's with about 15 un-removable snapshots on each.

Not quite sure where to go next and would appreciate a little assistance.

Thanks!