Unable to BackUp Database on SQL2005
There is a MS KB article on it which say it happens because


  
  The full-text catalog folder is either deleted or corrupted.
  You did not enable the database for full-text indexing.
  The database is restored from a Microsoft SQL Server 2000 database backup. Therefore, the folder of the full-text catalog in the database does not exist on the server where you restore the database.
  The instance of SQL Server 2005 that you are running was upgraded from SQL Server 2000. However, the full-text search service cannot be accessed during the upgrade.
  The database is attached from somewhere. However, you specify the incorrect location for the full-text catalog folder during the attachment.
  


The article also has a "workaround" section