On starting SQL Backup, the following error may occur:

This is caused by a corruption of the activity cache, which stores backup and restore history.

To fix this problem, close the SQL Backup graphical user interface (GUI) and delete the activity cache files from the machine running the SQL Backup GUI. By default these are stored in:

• %USERPROFILE%\Local Settings\Application Data\Red Gate\SQL Backup\Server Data (Windows 2003 and Windows XP), or 
• %LOCALAPPDATA%\Red Gate\SQL Backup\Server Data (Windows 2008, Windows 2008 R2, and Windows 7)

When you next open the SQL Backup GUI, the activity cache is populated from the msdb database and the SQL Server Compact database for each SQL Server that you have added to the GUI. This may take some time.