Ttgjyuk

I just updated from 16.04 to 18.04. Upon restarting I ended up at what appears to be the terminal. I was prompted for my login and password and got links to documentation, management, and support, as well as a little message about meltdown and spectre. Below that is:

[80.133769] Could not find key with description: [alphanumericstring]
[80.133922] could not find valid key in user session keyring for sig specified in mount option[samestringasabove]
[80.134129] Error parsing options; rc = [-2]

What is happening here and how do I get to my desktop? I am unsure if that key string should remain private so I left it out.

I had the same problem after upgrading to 18.04. The computer would boot into text terminal, and I could start the X Session with startx. I could resolve the problem with:
sudo apt-get install --reinstall gdm3

It seems this is a problem with the display manager and not necessarily linked to the ecryptfs bug. I think I was using lightdm before the update which is not the default.

I faced with this problem when activated Timeshift snaphots schedule (tool for making system backup and recovery). Backup's stole all free disk space, and after "sudo rm -rf /timeshift/snapshots" GUI booted automatically.

You are hitting this bug:
https://bugs.launchpad.net/ubuntu/+source/ecryptfs-utils/+bug/1718658

I have the same problem and I haven't found a way to fix it.

There are a few workarounds mentioned in the report but none of them worked for me.

Keep reading for what did work to get my system working again:

There is a chance that this is not really your problem. In my local instance this seemed to be an issue with the graphics drivers, as (after installing SDDM from the command line) I was seeing the mouse cursor, but I didn't see the log-in screen. I ended up in this question because the message you posted was the only error message I saw. However, if you look into /var/log/X11.0.log you will see that it fails to load libGL.so.1, and it tries to use Vesa. While digging into the issue I discovered that libglx-mesa0 libgl1-mesa-dri and some other packages were corrupted and had missing configurations and files

What I did to fix this:
I purged all xorg, xserver*, lightdm, gdm3, sddm, libglx*, libgl0, libgl1 and everything I could find related to the UI. Specifically around libGL.so.1 (apt-file find libGL.so.1) and then installed it again using apt.

Of course I ended up removing most of my system and restarted to install all packages from the ground, but at some point in this mess, I reinstalled libgl1-mesa-dri (+dependencies), sddm and kde-plasma-desktop, and I was finally able to boot into my system again. I understand this answer is quite chaotic, but this is what worked for me, and the real reason behind my issues was not related to the error message you are seeing, but to a broken mesa installation. This may be your case too.

As a side note I would like to warn you that at some point I stopped seeing errors about libGL.so.1 in my X logs, but it kept refusing to load the drivers anyway, (and SDDM didn't show the log-in screen either!). So I kept removing and reinstalling packages around mesa and libgl* until my installation started working again.

Good luck!