Dell Optiplex 360 Bios Update fails using custom UBCD5

One of several failed attempts at updating the BIOS of some Dell Optiplex 360 using a custom version of Ultimate Boot CD 5.3.3. I would like to know why it fails using this method. I also tried with the "Boot no UMB (defensive)" menu, disabling everything but CD-ROM support and french layout keyboard, but it still fails in the same way, on several machines. I ended up trying with another custom version made from UBCD 4.1.1, which works flawlessly on the very same machines (see https://www.youtube.com/watch?v=PxwVGcNvJdo )