You see this error message when trying to start a VM that has a .VHD disk
and a snapshot:

	Parent UUID {00000000-0000-0000-0000-000000000000} of the medium '...
	.vdi}' does not match UUID {1b3b7ea8-... .vdi} of its parent medium stored
	in the media registry

Solution: manually set the parent UUID field of the snapshot differencing disk.
Find the snapshot disk file and do:

vboxmanage internalcommands dumphd info : from the parent and the snapshot to get UUIDs
vboxmanage internalcommands sethdparentuuid : to set the parent of the errant snapshot to the proper vhd