There is indeed a bug with the release version of the SQL Backup GUI converter. Please download the patched version using the following URL:
ftp://support.red-gate.com/patches/sql_backup/SQBConverterGUI_7.4.100.zip
Replace the existing files in your SQL Backup installation folder with the files in the archive.
ftp://support.red-gate.com/patches/sql_backup/SQBConverterGUI_7.4.100.zip
Replace the existing files in your SQL Backup installation folder with the files in the archive.