Yeah, the FTP link no longer works. It looks like the process for disabling AES has changed slightly. See MIGR-5086963 for more details.
Take note in the IBM link above that you can enable AES now just by installing "UEFI version 1.10 or newer and restore the default settings in UEFI". I'm pretty certain that that applies to your old blades too so you can either disable AES on all your newer blades going forward or bite the bullet and enable AES on all your older blades (after upgrading the UEFI on them to 1.10 or newer). Going forward AES will always be enabled on newer blades I'm sure so I'd recommend enabling it on older blades whose UEFI couldn't support it when they first came out.
Blessings!