Where did you get that file from? We are currently upgrading a number of WAVE devices (294, 594 and 694), and we use waas-bios-installer-20a-19a-13a-k9.bin (the only one I found on CCO), and this works fine.
Also, have you tried doing this using a USB stick rather than FTP, much easier and quicker. Just copy the file to a USB stick, plug it in to the WAVE, and enter
copy usb install <filename>
don't forget to take the USB stick out before reloading though :)