Sorry you are experiencing this.
The error you are receiving is likely because the SQL Server Compact Database we use to hold the backup history information has hit the maximum size allowed: "The database file is larger than the configured maximum database size."
The simplest way to get the tool working again is to stop the backup service on the server issuing the warning, rename the data.sdf file, and then start the service again. This will create a new file.
This file is located on the server in the following location:
C:\\ProgramData\\Red Gate\\SQL Backup\\Data\\instance_name
Do you know how much history you're keeping? It's unusual for the DB to hit the maximum size, so maybe you haven't set it to ever delete any history. You can set the amount of history to delete in the File Management options of the server.
http://documentation.red-gate.com/displ ... t+options#
Alternatively you can use script in the following knowledge base article to manually delete the history.
http://documentation.red-gate.com/displ ... +manually#