There is a twain scanner driver conflict with another scanner:
- Disconnect the USB cable between the Macintosh and your Brother device.
- Log on as an "Administrator".
- Delete any scanner by another manufacturer (Macintosh HD/Library/Image Capture/TWAIN Data Sources)
- Reboot the Mac.
- When the Mac has rebooted, empty the Trash.
- Plug in the USB cable and try to scan again.