Cannot Delete Shadows - vssadmin

Hello,

Scheduled Backup cannot run because disk is full and when I try to delete older backups on Windows Server 2008 R2, vssadmin gives the following error with no relevant google results:

C:\Users\Administrator>vssadmin delete shadows /FOR=T: /oldest

vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool

(C) Copyright 2001-2005 Microsoft Corp.

Error: Snapshots were found, but they were outside of your allowed context.  Try

 removing them with the

backup application which created them.