Backup completed with warnings every other night.

The system is a Dell R720 running Microsoft Server 2012 with Hyper-V.  The backup is configured to run every night at 2:30AM and backup the System state, 2 VMs and Host Component.  Every other night the backups complete with a warning.

Time            Message          Description
11/12/2013  2:30 AM       Backup        Successful
11/11/2013  2:30 AM       Backup        Completed with warnings
11/10/2013  2:30 AM       Backup        Successful
11/9/2013    2:30 AM       Backup        Completed with warnings

When I check the details for the Completed with warnings, the System State Backup did not complete.  When viewing the list of all backed up files it stops at the same point for the System State Backup.  The error message is:
       System state           The operation ended before completion.  Detailed  error: The process cannot access the file because
                                     it is being used by another process.

The event log has the following Warning messages:

Warning 11/11/2013  3:01:45 AM Backup 519 None

Warning 11/9/2013  3:02:34 AM Backup 519 None

Warning 11/7/2013  3:02:06 AM Backup 519 None

Warning 11/5/2013  3:01:47 AM Backup 519 None

The detail appears to be the same for all of them as follows:

Log
Name:      Application

Source:    Microsoft-Windows-Backup

Date:       11/11/2013 3:01:45 AM

Event
ID:      519

Task
Category: None

Level:        Warning

Keywords:
    

User:         SYSTEM

Computer:     ems2013.EMS3.local

Description:

The backup operation that started at '2013-11-11T07:30:19.339260600Z' has
failed to back up volume(s) ''. Please review the event details for a
solution, and then rerun the backup operation once the issue is
resolved.

Event Xml:

<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">

  <System>

   <Provider Name="Microsoft-Windows-Backup" Guid="{1DB28F2E-8F80-4027-8C5A-A11F7F10F62D}" />

<EventID>519</EventID>

<Version>1</Version>

<Level>3</Level>

<Task>0</Task>

<Opcode>0</Opcode>

<Keywords>0x8000000000000000</Keywords>

<TimeCreated SystemTime="2013-11-11T08:01:45.917175200Z" />

   <EventRecordID>6685</EventRecordID>

   <Correlation />

   <Execution ProcessID="3028" ThreadID="5884" />

   <Channel>Application</Channel>

   <Computer>ems2013.EMS3.local</Computer>

   <Security UserID="S-1-5-18"
/>

 </System>

 <EventData>

    <Data Name="BackupTime">2013-11-11T07:30:19.339260600Z</Data>

    <Data Name="FailedVolumeNames">

</Data>

    <Data Name="ErrorCode">0x807800c5</Data>

    <Data Name="ErrorMessage">%%2155348165</Data>

 </EventData>

</Event>

I have tried changing the backup time to a later time and deleted the backup schedule and re-entered it.

I have been searching the forums but nothing seems to be related to my situation.

Any help would be greatly appreciated as the server is already in production.

Thanks,

Paul