SQL Server 2008 unable to backup due to Failed to flush the commit table to disk in dbid 11 due to error 2601
Do you happen to use the change tracking services provided by in SQL 2008 Enterprise?  If so, it's time to update your system.  Please try to avoid running RTM anywhere.  It's very risky from a security and sometimes stability point of view. :)

The following Microsoft Knowledge Base articles may be of use to you:



FIX: Backup fails in SQL Server 2008, in SQL Server 2008 R2 or in SQL Server 2012 if you enable change tracking on the database

This fix was first released in Cumulative Update 7 for SQL Server 2008 Service Pack 2.


  Symptoms:
  
  Error: 2601, Severity: 14, State: 1.  Cannot insert duplicate key row
  in object 'sys.syscommittab' with unique index 'Name'.  Error: 3999,
  Severity: 17, State: 1. Failed to flush the commit table to disk in
  dbid due to error 2601.




FIX: The sys.syscommittab system table is not cleaned up after the retention period for change tracking in SQL Server 2008

This fix was first released in Cumulative Update 11 for SQL Server 2008 Service Pack 1. 


  Symptoms:
  
  You enable the Change Tracking and Auto Cleanup properties for a
  database in Microsoft SQL Server 2008.
  
  You set the Retention Period and Retention Period Units properties.
  In this scenario, the sys.syscommittab system table for change
  tracking is not cleaned up after the specified retention period.