Problem, new in 16.04:
When I run the command
sudo mokutil --disable-validation to disable secure boot validation within Ubuntu, I am asked to type a password twice and then get the message
Failed to request new MokSB state
Google reveals very little about this obscure error message!
I cannot install nvidia drivers as a result - when the system reboots after the driver install I can't log in, and have to bring up a terminal to purge the drivers and reboot. I have never seen the post-boot interface for disabling secure boot (which I expected to see and was asked to set a password for). This sequence of events (no disabling of secure boot and then failed login) happens whether I select to install third party drivers (and "disable secure boot") during Ubuntu installation, or install them from the Additional Drivers menu, or install nvidia-364 from the graphics drivers ppa. (From what I can tell, Ubuntu is actually offering to disable secure boot validation within Ubuntu and leave the motherboard's secure boot setting unchanged - the terminology isn't helpful.)
I have an ASUS Sabertooth Mark 2 desktop with Nvidia GTX 970 that I dual boot with Windows 10 (using the BIOS boot override to choose the OS). Secure boot is enabled in the BIOS and I want to keep it that way for Windows at least. Ubuntu (15.10 and earlier) worked fine as the Shim (and grub?) were signed for secure boot, but no checking was done later in the boot process (when the nvidia drivers come into play). Ubuntu 16.04 newly enables checking throughout the boot process, but it's tripping up when it gets to the nvidia drivers.