The identity under which the scheduled backup is being run may be inadequate. See "Before you perform an image backup" in the client manual - particularly the italicized advisories. Another, but less likely possibility is that the environmental activity of the schedule is in some way itself getting involved with the volumes and thus interfering.
You don't specify your client level: as always, assure having the latest maintenance level applied for the given version/release. If you're stuck, run a client trace to expose just what it's having trouble with. Richard Sims
