[Desktop] Desktop keeps crashing, several important programs keep crashing

[PSPurgatory]

Hope this is the right place to turn to, because this is a bit of an emergency.
My desktop is completely broken. It's in this sort of looping state where it crashes, then restarts, then crashes and restarts again until it eventually stops trying and I end up with an unusable greyish-black screen.
Kate, Krita and Dolphin crash on startup and are completely unusable. Konsole crashed like two times under certain conditions but it seems to be fine. I'm able to launch Konsole using Ctrl+Alt+T.
Fortunately I have previously setup 2 spare user accounts, and upon logging in these two seem to be unaffected. It appears that this issue is isolated entirely within my main user account, in which I store all my files and applications. Other than that I don't really have any idea on what could the problem be. I am writing this on one of these spare profiles.
I've been happily using Linux for a couple months now but I can't consider myself confident enough in my abilities to solve this problem on my own. This is the first time something like this has happened to me. Help would be much appreciated.

Kubuntu 24.04
KDE Plasma version 5.27.12
Graphics platform X11
Ram: 16 gb
CPU: AMD Ryzen 5 2550H
GPU: AMD ATI Radeon Vega Series and NVIDIA GeForce GTX 1650 Mobile

[Snowhog]

Welcome to KFN.

Do you remember when this started? What preceded this situation? Do the '2 spare user accounts' use the same username as the FUBAR account?

[PSPurgatory]

I completely forgot to mention this but this jogged my memory. It started happening today (everything was fine yesterday) and at first the desktop itself was fine. Only the programs listed were broken. Then I restarted my computer thinking it was some small inexplicable hiccup and now I'm here.
If i remember correctly when at first Kate crashed the listed reason was a segmentation fault.
I should also mention that post-reboot (once the desktop was broken) I checked a bunch of program crash reports, and while I don't really understand them a bunch of lines mentioned the /lib/x86_64-linux-gnu/ directory. I don't know how useful this information could be.
Spare users are labelled 'Guest 1' and 'Guest 2', with matching usernames 'guest1' and 'guest2'. Also I don't know what 'FUBAR' stands for...

[Snowhog]

I don't know what 'FUBAR' stands for...

F'd up beyond all recognition. FUBAR (disambiguation)

With a reboot, and logging in with your regular user account, is the Desktop stable if you don't launch anything?

[PSPurgatory]

UPDATE: The situation has changed. Real quick I logged back in to my main account and randomly the desktop was fine Or at least seemed fine... I haven't clicked on it or interacted with it in any way, but it was at least visible and didn't constantly crash. Programs like Kate or Dolphin still crashed.
I tried to log out, then the following message appeared:
"
The screen locker is broken and unlocking it is not possible anymore.
In order to unlock it, switch to a virtual terminal (e.g. Ctrl+Alt+F2),
log in to your account and execute the command:

loginctl unlock-session 40

Then log out of the virtual session by pressing Ctrl+D and switch back to the running session (Ctrl+Alt+F1).
Should you have forgotten the instructions, you can get back to this screen by pressing CTRL+ALT+F40
"
Then I pressed Ctrl+Alt+F2 and logged into guest1.
This could be related to another issue posted in this forum recently.

[PSPurgatory]

Another update: Some time about an hour ago I tried logging back in to the main account and the desktop is crashing again. So I assume I'm back to square one.

[skyfishgoo]

sharing this discussion because it seem relevant here.

https://discuss.kde.org/t/background-briefly-shows-and-plasma-crashes-plasmashell-version-5-27-12/50408

[PSPurgatory]

^^^^^ i quickly skimmed this post, It does seem like it tackles a very similar issue! Excuse me for sort of drip-feeding information but at one point I dpkg in recover mode and it updated 3 packages involving the program 'apparmor', which is mentioned in the first error log in the post.
I've had my doubts before but maybe could this be caused by some sort of faulty update?

[Snowhog]

If you can get a running Konsole going, run sudo apt update && sudo apt full-upgrade

Note any actual Error messages.

[skyfishgoo]

do you use a chrome browser?

[PSPurgatory]

sharing this discussion because it seem relevant here.

https://discuss.kde.org/t/background-briefly-shows-and-plasma-crashes-plasmashell-version-5-27-12/50408

BIG UPDATE: I checked out this forum post again and ran this command on my main profile:
fc-cache -r
Everything now seems fully operational!
fc-cache is a program that "scans the font directories on the system and builds font information cache files for applications using fontconfig for their font handling". The parameter "-r" Erases all existing cache files and does a rescan. In a bunch of crash reports that I've seen the first or the second line did mention 'libfontconfig.so.1'. I'm guessing maybe there is some kind of vulnerability involved... The very last reply in the linked forum post says the issue could be because of "the latest Chrome". This could make sense, since I have Brave as my current main browser as well as Discord, which is apparently Chromium-based according to some online posts.
I highly recommend checking out this link in the quoted reply. If this really is some kind of faulty update/vulnerability then there might be more people struggling because of this.

[PSPurgatory]

do you use a chrome browser?

I was writing about this just now actually! I use Brave, which is supposedly based off of Chromium.

[PSPurgatory]

If you can get a running Konsole going, run sudo apt update && sudo apt full-upgrade

Note any actual Error messages.

Currently sudo apt update returns All packages are up to date.