PKCS Signature error/warnings running dmesg on Ubuntu Mate 18.04

I likewise have an Nvidia card using the proprietary Nvidia driver.

On first boot after upgrading from 17.10 to 18.04 by the message:

PKCS#7 signature not signed with a trusted key

was reported 3 times before reaching the login screen and the boot-sequence stalled. I could only boot in Recovery Mode. Disabling Secure Boot in the BIOS made no difference.

Having booted in Recovery Mode, however, I could select Resume normal boot from the action menu and a normal boot sequence then proceeded successfully.

I launched Software & Updates and opened the Additional Drivers tab. Under 17.10, my Nvidia graphics card driver had been been the proprietary one provided by the Ubuntu nvidia-driver-390 meta-package. Now, the card was not reported as using that proprietary driver, or the open-source xorg-xserver-video-noveau driver. It was shown as using a manually installed driver, and the usual proprietary and open-source driver options were unselectable.

I then established by:

dpkg -l nvidia-driver-390

that nvidia-driver-390 was no longer installed. So I installed it:

sudo apt install nvidia-driver-390

Then rebooted, and the boot sequence ran successfully and normally. After logging in I revisited Software & Updates -> Additional Drivers and now saw that my graphics card was reported as using the proprietary nvidia-driver-390 driver.

I was able to fix it it seems. Just make sure you delete absolutely everything related to nvidia(purge including all configs and i386 as well). Make sure dpkg -l | grep nvidia returns an empty result. Then go for:

sudo apt install nvidia-driver-396

(version can vary, of course)

It opens up a graphical interface inside your terminal at some point and proposes to add a signing MOK key. After I did that I rebooted and entered the key when prompted.