The screen locker is broken and unlocking is not possible anymore

[MartinDT]

apt list --upgradable
Listing... Done
apparmor/noble-updates 4.0.1really4.0.1-0ubuntu0.24.04.8 amd64 [upgradable from: 4.0.1really4.0.1-0ubuntu0.24.04.7]
dmidecode/noble-updates 3.5-3ubuntu0.2 amd64 [upgradable from: 3.5-3ubuntu0.1]
dnsmasq-base/noble-updates 2.91-0ubuntu0.24.04.1 amd64 [upgradable from: 2.90-2ubuntu0.4]
libapparmor1/noble-updates 4.0.1really4.0.1-0ubuntu0.24.04.8 amd64 [upgradable from: 4.0.1really4.0.1-0ubuntu0.24.04.7]
libaudit-common/noble-updates,noble-updates 1:3.1.2-2.1ubuntu0.1 all [upgradable from: 1:3.1.2-2.1build1.1]
libaudit1/noble-updates 1:3.1.2-2.1ubuntu0.1 amd64 [upgradable from: 1:3.1.2-2.1build1.1]
libpciaccess0/noble-updates 0.17-3ubuntu0.24.04.3 amd64 [upgradable from: 0.17-3ubuntu0.24.04.2]
xserver-xorg-core/noble-updates 2:21.1.12-1ubuntu1.8 amd64 [upgradable from: 2:21.1.12-1ubuntu1.6]

[Gabriel M]

I managed to fix the issue where Plasma would crash upon login, leaving only a black screen and the cursor. The problem stemmed from two desktop widgets—"org.kde.MinimalMusic" and "org.zayronxio.vector.clock"—as well as an issue involving Fontconfig and the graphics engine (specifically `libfontconfig.so.1` and `libQt5XcbQpa.so.5`). The failure occurs right when Plasma attempts to process fonts or render a visual component (such as `QQuickLabel`) during startup; this can be caused by a corrupted user font cache or a font configuration file that is breaking Qt.

To resolve this problem once and for all, we do the following. We are going to regenerate the system font cache and completely clear the user font temporary files:

1. Let's ensure there are no Plasma processes running in the background:

killall plasmashell

2. Clear the user icon cache, as it may have become corrupted and caused segmentation faults:

rm -rf ~/.cache/fontconfig

rm -rf ~/.cache/icon-theme.cache

3. We regenerated the font cache at the system level:

sudo fc-cache -f -v

4. We restart Plasma:

plasmashell &

5.And if we see that Plasma no longer crashes, we perform a full system reboot so that everything loads normally:

sudo reboot

This is what finally worked for me; I hope it helps those who might be having a similar problem.

I was able to do all this because I had access to the terminal from the desktop using Ctrl + Alt + T; if you don't have access to the terminal, try Ctrl + Alt + F3, but skip step 4 and go straight to step 5.

Greets.

[claydoh]

To resolve this problem once and for all, we do the following. We are going to regenerate the system font cache and completely clear the user font temporary files:

Do you use Google Chrome?

https://www.reddit.com/r/Kubuntu/com...rome_corrupts/

https://issues.chromium.org/issues/565052667
https://issues.chromium.org/issues/565132857

Until Google updates Chrome (today?) it might be worth using a different browser for a short while.

[Gabriel M]

Do you use Google Chrome?

https://www.reddit.com/r/Kubuntu/com...rome_corrupts/

https://issues.chromium.org/issues/565052667
https://issues.chromium.org/issues/565132857

Until Google updates Chrome (today?) it might be worth using a different browser for a short while.

Thanks for the tip. You're right—the problem started when I launched Chrome. I'm not sure if Opera is also an issue, given that it's based on Chromium (I only use those browsers for testing), but Firefox is my main browser. Like you said, it's best to wait until those issues are fixed.

[claydoh]

I'm not sure if Opera is also an issue, given that it's based on Chromium

This seems specific to Chrome proper, not Chromium so I think the derivative browsers probably aren't causing this.

[skyfishgoo]

there was at least one user caught up in this who reported using Brave as their browser

[MartinDT]

how these Chrome posts relate to my screenlocker issue?

[Snowhog]

Warning: The information provided below is AI generated and should not be taken as gospel or authoritative. It may or may not be correct in whole or in part. But it may lead to a cause and solution.

Searched Google with Kubuntu 24.04 kf.kirigami: Failed to find a Kirigami platform plugin Segmentation fault (core dumped) and Google AI came back with:


The Segmentation fault (core dumped) error alongside kf.kirigami: Failed to find a Kirigami platform plugin on Kubuntu 24.04 usually happens because a KDE application is missing its required QML/Kirigami environment files, or there is a mismatch in your Qt environment variables.

Quick Fixes

• Install missing Kirigami packages:
  Open your terminal and run the following command to ensure all necessary Kirigami 2 and 3 components are installed: bash
  
  sudo apt update && sudo apt install qml-module-org-kde-kirigami2 qml-module-org-kde-kirigami libkf6kirigami6 libkf5kirigami2
  
  Use code with caution.
• Reset conflicting environment variables:
  If you are running the application with sudo, or if you have custom Qt styles set (like QT_QPA_PLATFORMTHEME), it can crash Kirigami apps. Try running the application by explicitly clearing the platform theme for that command: bash
  
  QT_QPA_PLATFORMTHEME= application-name
  
  Use code with caution.
  
  (Replace application-name with the actual name of the app you are trying to launch).
• Fix Snap or Flatpak isolated environments:
  If this error happens with a specific Snap or Flatpak application, the app cannot see your system's desktop theme.
  • For Flatpak, update your runtimes: flatpak update
  • For Snap, try switching the app to the candidate channel or reinstalling it, as it is likely a packaging bug specific to that application on Ubuntu 24.04.

To help narrow down the root cause, could you tell me:

• What application are you trying to launch when this crash happens?
• Does this happen when running the app as a normal user, or only with sudo / root privileges?
• Was this application installed via apt, Snap, or Flatpak?

kscreenlocker


When kscreenlocker (specifically kscreenlocker_greet) throws a Segmentation fault (core dumped) coupled with kf.kirigami: Failed to find a Kirigami platform plugin, the lock screen architecture is crashing when trying to render its visual components. [1, 2]
Because KDE locks down the system for security, a crashing lock screen leaves you stranded on a broken screen. [1, 2]

---

Step 1: Recover your current session (If you are locked out)
If your screen is currently broken or black, you need to manually force an unlock: [1, 2]

• Press Ctrl + Alt + F2 (or F3, F4) to switch to a virtual terminal (TTY).
• Log in with your standard Ubuntu username and password.
• Run the following command to force-unlock your graphical session: bash
  
  loginctl unlock-session $(loginctl list-sessions | awk 'NR==2 {print $1}')
  
  Use code with caution.
• Press Ctrl + Alt + F1 (or F7) to return to your desktop. [1, 2]

---

Step 2: Fix the Root Cause
The crash is typically caused by a corrupted QML cache, missing Virtual Keyboard modules, or theme permission issues. Run these steps sequentially from a terminal: [1, 2, 3]

1. Clear the Plasma / QML Cache
The underlying Kirigami framework caches user-interface elements. A bad cache often triggers a segmentation fault upon a version check. Wipe it out completely: [1]

rm -rf ~/.cache/qmlcache/ rm -rf ~/.cache/plasmashell/ rm -rf ~/.cache/kscreenlocker/

Use code with caution.

2. Install the Missing Qt Virtual Keyboard Component
Even if you do not use an on-screen keyboard, kscreenlocker imports the virtual keyboard component into the main lock screen script. If it's missing or partially updated, it crashes the greeter instantly. [1]

sudo apt update sudo apt install qtvirtualkeyboard-plugin qml-module-qtquick-virtualkeyboard

Use code with caution.

3. Reinstall and Re-register Kirigami
Ensure the Kirigami plugins are properly registered in your system architecture:

sudo apt install --reinstall qml-module-org-kde-kirigami2 qml-module-org-kde-kirigami libkf6kirigami6 libkf5kirigami2

Use code with caution.

4. Fix Broken Home Permissions
If you have ever launched a GUI application using sudo rather than pkexec, root may have claimed ownership over configurations in your home directory. If kscreenlocker cannot read your theme preferences, it throws a permissions error and segfaults. [1, 2]

sudo chown -R $USER:$USER ~/.local ~/.config ~/.cache

Use code with caution.

---

Step 3: Test your solution
Instead of risking getting locked out again, test the greeter in a safe sandboxed window inside your active desktop environment: [1, 2]

/usr/libexec/kscreenlocker_greet --testing

Use code with caution.

• If a tiny window pops up displaying your lock screen theme, the issue is resolved. You can close it.
• If it crashes immediately with a Segmentation fault again, check the terminal output for lines following the Kirigami error to see if a specific .qml file or a physical graphics driver (libQt5Gui / Nvidia) is triggering the crash. [1, 2]

If the test command still crashes, please run:

journalctl -b 0 | grep kscreenlocker

Let me know what the last 3 to 4 lines say—specifically if it references a graphic driver (like Nvidia/Mesa) or a specific third-party lockscreen theme.

[skyfishgoo]

you haven't provided any additional information for anyone to go on

a list of packages is all i see from you.

are you still having the issue?

what exactly is happening when you try to reboot?

what is the output of kinfo?

[MartinDT]

okay, thanks for the information, I will proceed with what Snowhog suggested

in the mean time

martin@martin-inspiron153530:~$ kinfo
Operating System: Kubuntu 24.04
KDE Plasma Version: 5.27.12
KDE Frameworks Version: 5.115.0
Qt Version: 5.15.13
Kernel Version: 6.8.0-142-generic (64-bit)
Graphics Platform: offscreen
Processors: 12 × 13th Gen Intel® Core™ i5-1335U
Memory: 15.3 GiB of RAM
Graphics Processor: Mesa Intel® Iris® Xe Graphics
KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = kcmshell5 path = /usr/bin pid = 206600
KCrash: Arguments: /usr/bin/kcmshell5 kcm_about-distro --args dump
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi

[1]+ Stopped kinfo

[MartinDT]

so .. I restarted my Kubuntu and ended up in login screen, logged in and ended up in black screen with mouse cursor and no bottom tool bar
I tried what Gabriel M suggested:

1. killall plasmashell - nothing was running
2. rm -rf ~/.cache/fontconfig DONE
3. rm -rf ~/.cache/icon-theme.cache - I didn't have this folder, ~/.cache/icon-cache.kcache only, so I skipped this step
4. sudo fc-cache -f -v DONE
5. plasmashell & - DONE, but with error

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

Aborted (core dumped)

Pressed Ctrl-C and
6. sudo reboot

After reboot, I was able to login into desktop, plasma works (no black screen), but still have the same issue with screenlocker and some apps are not able to start like KDiscover or Timeshift

[MartinDT]

okay, it looks like this solved the issue

I have updated google-chrome first (there was an update available, version 154.0.8037.92 (Official Build) (x86_64)

killall -9 krunner plasmashell kscreenlocker_greet
rm -rf ~/.cache/fontconfig/
sudo fc-cache -f -v
rm -rf ~/.cache/plasma*
rm -rf ~/.cache/krunner*
rm -rf ~/.cache/ksycoca5*
kbuildsycoca5 --noincremental
plasmashell &