VirtualBox error after deleting snapshot

Problem is that once a snapshot file is missing, Virtualbox will not allow you to delete the snapshot from the GUI. To "force-delete" the snapshot you will have to let Virtualbox believe the file is not missing. So just copy an existing one and rename it to the missing file's name. Then u need to set for each new file its correct UUID and its parent UUID. For example:

VBoxManage.exe internalcommands sethduuid "path\{c87e9357-5d1a-4d00-84d8-7b43293ab92e}.vmdk" c87e9357-5d1a-4d00-84d8-7b43293ab92e
VBoxManage.exe internalcommands sethdparentuuid "path\{c87e9357-5d1a-4d00-84d8-7b43293ab92e}.vmdk" c9e9b5a8-fb3f-42f8-aeb7-4458bf92f625

All the missing filenames and their UUIDs you can find in the Virtual Media Manager.