Exchange backup exec Job Sometimes Failed due to VSS and Sometimes succeeded , Without any solution.

Dears ,

We backup DAG exchange server 2010  Using backup exec 2010 R3 and updated With SP2  and  recent Hotfixes

The backup job backup the passive copy of the database .

The backup job is unstable , Sometime Succeeded and simetime failed with the following error related to VSS

1. Exchange VSS Writer (instance b7912ba8-f300-417b-95ee-a42c19ec93e1:10) failed with error code -2403 when preparing for Snapshot.
2. Exchange VSS Writer (instance b7912ba8-f300-417b-95ee-a42c19ec93e1:10) failed with error code 1295 when processing the post-snapshot event.
3. Exchange VSS Writer (instance b7912ba8-f300-417b-95ee-a42c19ec93e1:10) failed with error code 1295 when processing the backup completion event.
4. Exchange VSS Writer (instance 0ffb1558-aae8-4c1e-b49e-a81b72e392b1:10) failed with error code -2403 when preparing for Snapshot.
5. Information Store (14908) Shadow copy instance 10 aborted.

We reboot the servers that are members of DAG but the job is still  unstable.

I hope to provide me with your ideas Why that happen ?

Thanks