the Volume Shadow Copy Service operation to create a shadow copy of the volumes being backed up failed with following error code '2155348129'

hi

I have Windows server 2008 r2 with Hyper-v ( two virtual machine DC,ISA),but when i want create back up with Windows Server Back up on external storage i see this error:

Failed to create the backup of virtual machine 'ISA2'. (Virtual machine ID 1239204B-5195-40DB-805E-D404D753A0DA)

but when i stop ISA VM i can create back up.

what can i do?

help