Nice screenshot which of course is always helpful (not many people go to the trouble of doing it), I just hope you don't mind exposing your console serial number to the world!
I got this same error after selecting the backup and patch option, so I ended up doing the dosflash manually and everything seems to be working well. Thank you for your efforts.
I whacked the USB stick in my computer, went in to the backup folder for my serial number and there was indeed a 256k file there dumped by dosflash, except it was called:
BENQ-OFW.BIN
and not
BEN-OFW.BIN like the iprep software seems to be expecting
I think I had a similar issue trying to use the flash option.
It is possible a simple search and replace by Klutsch will fix this particular problem.
HTH
bwoats