Unable to copy the virtual machine disk using the VMware VixDiskLib.

Hello,

I have created seprate job for each VM through Vcenter, 

Randomly backup for several machine fails with above error and work at other time.

My media server is Win 2003 / BE 2012 SP2, got ESX 4.0 and 5.1 in a cluster

but backup fails / work for randomly.

Thanks