I'm having the same problem deleting snapshots.
VBox Version 5.0.14 r105127
I have attached two (2) files:
1. a screenshot of the snapshot list [inline]
2. the entire .vbox file for the VM
Two snapshots cannot be deleted. They are the last two on the list:

- Snapshots (screen cap)
- Capture.JPG (27.17 KiB) Viewed 4253 times
name="4.8.00 68428 UPG (bad.1)"
name="4.8.00 68428 UPG (bad.2)"
--- BEGIN: deletion error message ---
Failed to delete the snapshot 4.8.00 68428 UPG (bad.1) of the virtual machine REF_ubuntu_64-bit.
Storage for the medium 'D:\VirtualBox VMs\REF_ubuntu_64-bit\Snapshots\{b1126e1c-27f6-4474-86bd-57a6ec3c5aed}.vdi' is being deleted.
Result Code: VBOX_E_INVALID_OBJECT_STATE (0x80BB0007)
Component: MediumWrap
Interface: IMedium {4afe423b-43e0-e9d0-82e8-ceb307940dda}
--- END: deletion error message ---
I see the same error message when I try to delete the other snapshot, "4.8.00 68428 UPG (bad.2)".
One of the two snapshots cannot be restored:
--- BEGIN: restoration error message ---
Failed to restore the snapshot 4.8.00 68428 UPG (bad.2) of the virtual machine REF_ubuntu_64-bit.
Assertion failed: [m->state == MediumState_LockedRead] at 'F:\tinderbox\win-5.0\src\VBox\Main\src-server\MediumImpl.cpp' (4411) in long __cdecl Medium::i_createDiffStorage(class ComObjPtr<class Medium> &,enum __MIDL___MIDL_itf_VirtualBox_0000_0000_0074,class MediumLockList *,class ComObjPtr<class Progress> *,bool).
Please contact the product vendor!
Result Code: E_FAIL (0x80004005)
Component: MediumWrap
Interface: IMedium {4afe423b-43e0-e9d0-82e8-ceb307940dda}
--- END: restoration error message ---
The .vbox file is attached (with a TXT extention to get past the file-type filter).