Hi,
Let me try,
1)from cmd prompt check for the VSS writer status by VSSadmin list writers, all the writers should give the "state: Stable and Last error: No error"
if any writer status is different than this then the the server of which the backup is failing should be rebooted to fix the issue with the writers (in most cases)
The ideal vss writers status should be as below for a successful backup.
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001 Microsoft Corp.
Writer name: 'System Writer'
Writer Id: {e8132975-6f93-4464-a53e-1050253ae220}
Writer Instance Id: {00450973-1f3a-4afd-84b6-b1257e42f2e0}
State: [1] Stable
Last error: No error
Writer name: 'Registry Writer'
Writer Id: {afbab4a2-367d-4d15-a586-71dbb18f8485}
Writer Instance Id: {d9a11119-af36-40fc-8f4f-cc0df2409bf6}
State: [1] Stable
Last error: No error
Writer name: 'Event Log Writer'
Writer Id: {eee8c692-67ed-4250-8d86-390603070d00}
Writer Instance Id: {d9d2d620-8ff1-438f-a87e-63301d7746bc}
State: [1] Stable
Last error: No error
Writer name: 'COM+ REGDB Writer'
Writer Id: {542da469-d3e1-473c-9f4f-7847f01fc64f}
Writer Instance Id: {f6be985c-8c1c-4e22-992f-b8c9d283679c}
State: [1] Stable
Last error: No error
Writer name: 'IIS Metabase Writer'
Writer Id: {59b1f0cf-90ef-465f-9609-6ca8b2938366}
Writer Instance Id: {9506aec4-e43c-4827-a4af-844e8209042c}
State: [1] Stable
Last error: No error
Writer name: 'WMI Writer'
Writer Id: {a6ad56c2-b509-4e6c-bb19-49d8f43532f0}
Writer Instance Id: {13702ed5-47c3-426f-94f9-db0a00c8a1b6}
State: [1] Stable
Last error: No error
Writer name: 'BITS Writer'
Writer Id: {4969d978-be47-48b0-b100-f328f07ac1e0}
Writer Instance Id: {6ed1de67-92a1-4e71-bf8c-d6c846354271}
State: [1] Stable
Last error: No error
2) make the VSS service startup type in services.msc to 'Manual'