Which logs shall I post to pastebin for an analysis?

The X.Org log, /var/log/messages & /var/log/syslog.

As long as you keep your wits about and don't do stupid things, like browsing unsafe websites and stuff, ASCII will work just fine. But in time you'll notice stuff does get outdated and no longer supported and websites may no longer work as you'd expect, or at all. But this is Linux: grab the source of your $browser, configure, compile, build and install it and you're done.   It's a pity Beowulf is such a PITA, but the underlying issues are not the fault of the Devuan dev's but lay with Debian or perhaps elsewhere upstream.

One thing that was different this time, thought, is that I had just downloaded updates and a couple of them were related to X. Dunno what the fixes were supposed to be. Maybe someone with better knowledge of where to look up recent patches can check on that. Since I had not rebooted/restarted X, that is probably a potential source of problems.

So, I did restart it after having to use the hard power switch, and fiddled with it a bit. Had also just installed Chromium, which does not complain about the missing keyring with Cinnamon, since the keyring is present. It was nice to use another open source browser for a bit.

Since this happened yesterday, I'm sure some of the logs will be archived and new ones started when I boot it again. Which logs shall I post to pastebin for an analysis?

I also tried removing the intel driver on a different laptop. Granted, this one hasn't had the bug, but I thought it might have better performance using the modesetting driver if that one is updated more often. Instead, it didn't even boot to the desktop! It stopped at the command prompt and I had to re-install the intel driver to get the desktop back. Here's the thing -- it is also using the i915 driver??? But the kernel driver is apparently not the same thing as the video driver. It's a Gateway laptop from ~2011, five years older than the HP. And it's running Devuan Ascii with XFCE. The modesetting driver apparently doesn't recognize the video subsystem.

Which logs shall I put on pastebin?

The logs aren't very large yet, since this is such a new system, but I really don't know what to look for. Grep for "Warning"? Or "Error"? Or something else?

Upload them to a pastebin service and share the link here, I'll have a look at them.

Did you try the modesetting driver yet?

The logs aren't very large yet, since this is such a new system, but I really don't know what to look for. Grep for "Warning"? Or "Error"? Or something else?

I'll have to try Ctrl-Alt-F1 if it happens again. I hadn't heard of that hot-key combination. I think it was Ctrl-Alt-Backspace that forces a desktop environment to quit? I didn't think to try that at the time, but will remember it next time along with Ctrl-Alt-F1....

There are a number of text consoles, accessed by ctrl-alt-F1, ctrl-alt-F2, etc. ctrl-alt-F7 should take you back to the GUI if that's working (it might be some other combination such as ctrl-alt-F6 so it's worth testing before you need it in anger).

And did the logs show anything from the last time it happened? Although I've had systems lock up or suddenly reboot with nothing interesting in the logs before the messages from the system coming back up.

Chris

create that directory

^ This. Or read the xorg.conf man page to find out where else you can place the file for it to be read.

Some NVIDIA cards require non-free firmware to work properly with the nouveau drivers.

The Intel DDX driver (xserver-xorg-video-intel) hasn't been updated properly in a long time and is known to be buggy, which is why the package information recommends to use the modesetting driver instead.

EDIT: does the lspci -k command show a kernel driver in use line for the video card? You haven't included it in your post but it should show i915.

I'm at a loss how to 'fix' the problem

You and everyone else, unless you can gather more information on what is actually going wrong. Hence the suggestion to check logs and system responsiveness outside the GUI by dropping to a console or connecting over SSH when this happens.

Yes, the video drivers seems to be the problem. When I saw something like this on a desktop with a nVidia-based video card using Devuan and XFCE a couple years ago, I gave up on Devuan and tried MX Linux. And it had the same problem... But when I installed the proprietary nVidia driver in MX, the problems went away. Now, this problem has something to do with Intel "HD Graphics" rather than an nVidia card, and the driver is apparently already the Intel driver? They published the source and Linux distros can just build it into their default config? So I'm at a loss how to 'fix' the problem this time. All I can do for now is try disabling the Intel part of X, I guess.

I'll have to try Ctrl-Alt-F1 if it happens again. I hadn't heard of that hot-key combination.

On key combos, it might also be worth enabling magic sysrq (IIRC it's restricted by default), if all else fails that usually gets you a console (or at least a memory dump) so you can see what's going on.

But yeah, I'm with HoaS, I recon it's probably a video driver problem.

how would I go about removing the xserver-xorg-video-intel part of X?

# apt purge xserver-xorg-video-intel

# apt install xserver-xorg-video-intel

EDIT: or just keep the package installed and create a file at /etc/X11/xorg.conf.d/00-modesetting.conf with this content:

Section "Device"
   Identifier "Intel Graphics"
   Driver "modesetting"
EndSection

Remove the file to revert to the Intel DDX driver.