Ubuntu – Ubuntu randomly, suddenly logs out the user session back to login

Since the last couple days, I'm suffering a killer bug on my Ubuntu 18.04 that never had before.

I'm working on my desktop session, and suddenly, randomly, the screen turns black and then the login screen appears. I have to login again, and a blank session starts (that means, all my previously opened apps, programs, windows – are gone, like if I did a fresh boot).

This does not happen always, but too often, enough to feel like it's a Russian Roulette. In fact, on the last time this bug happened, I could not even get to the login screen again – just the typical "/dev/sda1 clean xxxxx blocks…" message appearing on the top and nothing else, HDD without activity and nothing happened so I had to reboot pressing CTRL+ALT+SUPR twice.

Specs of my system are:

• CPU: AMD FX8320

• GPU: Nvidia GTX970 (using privative drivers provided by Ubuntu)

• Ubuntu: 18.04, vanilla/original flavour

• 2 screens (one with HDMI, one with VGA) – This bug happens since I changed my old secondary screen, connected using DVI-D, with my new secondary screen, connected using VGA.

Apart from all that, I have nothing weird installed that I'm aware of, but I edited the monitors.xml file on gdm3 to show the login screen on my HDMI monitor instead of the VGA one, by running sudo cp ~/.config/monitors.xml /var/lib/gdm3/.config.

However, I've been having this problem before I did this modification, but maybe it avoids getting me back to the login screen, so I restored the original file just in case.

Output from ls -al /var/crash:

total 89936
drwxrwsrwt  2 root  whoopsie     4096 mar 19 10:47 .
drwxr-xr-x 14 root  root         4096 jul 25  2018 ..
-rw-r-----  1 david whoopsie 92078339 mar 18 17:43 _opt_teamviewer_tv_bin_TeamViewer.1000.crash

Output from sudo lshw -C video:

  *-display
       descripción: VGA compatible controller
       producto: GM204 [GeForce GTX 970]
       fabricante: NVIDIA Corporation
       id físico: 0
       información del bus: pci@0000:01:00.0
       versión: a1
       anchura: 64 bits
       reloj: 33MHz
       capacidades: pm msi pciexpress vga_controller bus_master cap_list rom
       configuración: driver=nvidia latency=0
       recursos: irq:48 memoria:fd000000-fdffffff memoria:c0000000-cfffffff memoria:d0000000-d1ffffff ioport:e000(size=128) memoria:c0000-dffff

Output from ls -al ~/.local/share/gnome-shell/extensions:

total 32
drwxrwxr-x 8 david david 4096 feb  5 16:06 .
drwx------ 3 david david 4096 mar 25 11:49 ..
drwxrwxr-x 3 david david 4096 oct 29 18:02 alternate-tab@gnome-shell-extensions.gcampax.github.com
drwxrwxr-x 4 david david 4096 sep 23  2018 clipboard-indicator@tudmotu.com
drwxrwxr-x 6 david david 4096 nov  7 21:26 gnomeGlobalAppMenu@lestcape
drwxrwxr-x 4 david david 4096 feb  5 16:06 openweather-extension@jenslody.de
drwxrwxr-x 2 david david 4096 oct 30 21:19 topIcons@adel.gadllah@gmail.com
drwxrwxr-x 6 david david 4096 nov  7 21:29 unite@hardpixel.eu

Nvidia drivers installed: 390.116 (latest version provided by vanilla Ubuntu 18.04 repositories)