This issue occurs during the GRT enabled backup of Exchange Server 2007 running in CCR environment.

Following Event Id's are seen in the Event Viewer while investigating the cause for the issue.

Event Source: ESE
Event Category: Online Defragmentation
Event ID: 704
Description:
MSExchangeIS (5564) Mailbox SG: Online defragmentation of database 'M:\Mailbox database\Mailbox SG\DB.edb' was interrupted and terminated. The next time online defragmentation is started on this database, it will resume from the point of interruption.

Event Source: MSExchangeIS Mailbox Store
Event Category: Background Cleanup
Event ID: 9871
Description:
There is an online maintenance overlap for database "Mailboxes SG\DB". Please revise the online maintenance schedule.

Event Source: MSExchangeIS Mailbox Store
Event Category: General
Event ID: 1221
Description:
The database "Mailboxes SG\Mailboxes DB" has 493 megabytes of free space after online defragmentation has terminated.

Cause:
Virtual File Filter (VFF) communicates with ESE( Extensible Storage Engine) during the GRT- enabled backup operation. Performing Online Store maintenance while running backups can also cause performance issue and it may also result in job failure with following error.
Unable to complete the operation for the selected resource using the specified options for the following reason: VFF Open Failure.  This can be caused by low memory or disk resources.

Solution:

Change the backup schedule to avoid conflicts with online store maintenance. By default online store maintenance is scheduled to perform between 1 am - 5 am.

Note: Offline defragmentation would be required if changing the backup schedule doesn't help resolve the issue. But ensure after offline defragmentation, follow the correct backup schedule to avoid conflicts.
This issue is also acknowledge by Microsoft. To know more about it, please refer to the following Microsoft KB article.
 
Event ID 1221 reports less free space than should be the case
 http://support.microsoft.com/kb/555323

 http://technet.microsoft.com/en-us/library/aa997972(EXCHG.80).aspx