Backup error: Failed to flush the commit table to disk
This usually happens when you have enabled change tracking on the database. A fix was released some time back:

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

As it says there:


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


The latest (and final) Service Pack 4 for SQL Server 2008 would also contain this fix.