I've seen this one a few times - it's caused by the existence of a "lockfile" on the client-side activity history database that the SQL Backup Console uses. Since you can only have one console per user session, it assumes that if there is a lockfile present, the user is already running SQL Backup Console.
Please check %userprofile%\\appdata\\local\\red gate\\sql backup for a file called lockfile.pid. If it exists, delete it or check that it is not being locked by an anti-virus scan. If not, ensure you have full rights to the folder.