Hi Ben - the reason you can't currently delete this snapshot is a quirk of the storage backend that we use, and part of the reason that we can offer teams quite a large storage quota.
The way that it works is that each snapshot is dependent on the parent volume, and we store only the difference between the snapshot and the parent. Equally, if you create a new volume from a snapshot, it is dependent on the parent snapshot, which is in turn dependent on the parent volume.
So - in your case, you created a volume, then a snapshot, then a new volume (Newton's Apple) from the snapshot. Because Newton's Apple is dependent on the snapshot, you can't delete the snapshot (utterly unhelpful error messages aside - apologies!).
I'll flatten your Newton's Apple volume and remove the dependency on the preceding volumes and snapshots - you should then be able to delete both.