1. Check to see if the SQL Server instance is running in Full Recovery Mode.
  2. Shrink the database or purge the log file of the transaction data and run a new backup.
  3. Verify the available space on the hard drive and check it against the size of the MDF and Log Files.  See How to find the location of a SQL Server database.
  4. Request another backup and verify that it was created from the Blackbaud Management Console.