If Plasma started crashing after a Kubuntu 26.04 update, do not assume that clearing font caches will fix it. Check which process is crashing and whether its backtrace contains FcCharSetHasChar, then match the evidence to the reported trigger: the fonts-katex package, a possible font-cache startup race, or a Chrome 154 association. These are distinct reports, not one confirmed universal bug or an established official fix.
What is known about the Kubuntu 26.04 crashes?
Kubuntu 26.04 ships Plasma 6.6, Qt 6.10.2, KDE Frameworks 6.24.0 and Linux kernel 7.0, according to the Kubuntu 26.04 release notes. Plasma Wayland is the default supported session. Reported incidents mention Plasma 6.6.6 and kernel 7.0.0-34, but installed versions can change as updates arrive; check your own system before comparing it with a report.
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At least three possible fontconfig-related paths have been discussed. One Ubuntu bug report links font matching involving fonts-katex to crashes; an individual troubleshooting article describes a race while a font cache is being regenerated; and community reports associate some crashes with running Chrome 154. The available reports do not establish that these share a root cause, affect every Kubuntu system, or have the same remedy.
What FcCharSetHasChar tells you
A backtrace containing FcCharSetHasChar and fontconfig frames is a useful clue to investigate font handling. It does not, by itself, identify whether the trigger is a particular font package, cache timing, a browser, or another factor. Match the surrounding evidence before changing packages or startup behavior.
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Collect evidence before trying a fix
First establish what is actually failing. A plasmashell crash, a screen-locker failure, and a different application crash call for different investigations. Note what changed immediately before the first failure: a system update, a font-related package, or a browser update or first launch. Also record whether you are using Wayland or X11 and your installed Kubuntu, Plasma, Qt, and kernel versions.
Check crash records and warnings
Run these commands in a terminal to inspect recent core dumps and user-session warnings:
coredumpctl list --since today --no-pager
coredumpctl info plasmashell --no-pager
journalctl --user -b --no-pager -p warningIn the crash information, look for FcCharSetHasChar and related fontconfig frames. Exact output varies by system. If the trace points to another component, investigate that component rather than applying a font-cache workaround.
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The Ubuntu report says that two Kubuntu 26.04 installations had fonts-katex installed and that fc-match sans selected a KaTeX WOFF font. To check the installed package and font match, run:
dpkg -s fonts-katex
fc-match sans
fc-match -v sansThe dpkg command reports package status; the fc-match commands show which font fontconfig selects. Unexpected output is a reason to investigate this path, not proof that the package caused your crash. The report lists Plasma 6.6.6, Qt 6.10.2, fontconfig 2.17.1, and kernel 7.0.0-34 on its affected machines; those versions describe that report, not a general diagnosis.
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Check whether a browser update is relevant
Record your Chrome version and whether Chrome 154 had been run before the crash began. A Chromium issue search extract associates crashes on Ubuntu 26.04 with Chrome 154 and FcCharSetHasChar; KDE community discussion also raises Chrome or Brave as a possible clue. The available evidence is not enough to establish that all Chromium-based browsers are affected or that the browser is the cause on your machine.
Choose a remedy only when its symptoms match
If fonts-katex and the font match fit the report
An Ubuntu bug report filed on 2026-09-23 describes two Kubuntu 26.04 installations where plasmashell crashed in fontconfig, with FcCharSetHasChar in the backtrace and fc-match sans selecting a KaTeX WOFF font. The reporter’s workaround was to remove fonts-katex, clear both system and user fontconfig caches, and rebuild the system cache:
sudo apt purge fonts-katex
sudo rm -rf /var/cache/fontconfig/*
rm -rf ~/.cache/fontconfig
sudo fc-cache -fPurging a package changes installed software. Consider this sequence only if the package is installed and your font-matching results and crash trace resemble the report; do not remove it just because Plasma crashed after an update. The reporter said clearing only the user cache briefly fixed one login, but the two reported systems required clearing the system cache too. This is a workaround reported for those machines, not a verified general fix.
If the evidence fits a font-cache startup race
A separate troubleshooting article describes Plasma reading a font cache while another process regenerates it after login, producing a crash in FcCharSetHasChar. Its author reports success on a Kubuntu 26.04 system running Plasma 6.6.6 and kernel 7.0.0-34 after scheduling fc-cache before the Plasma shell service. That explanation and result are the author’s report, not an official Kubuntu diagnosis.
Do not apply the startup-order change unless the crash trace implicates fontconfig. It changes user-session service behavior, and the cited account does not establish that it is appropriate for other machines. The article also describes a temporary recovery sequence that clears user and system font caches, rebuilds them, resets the failed user service, and restarts Plasma; use its exact procedure only after confirming the diagnosis.
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The article provides a user-level systemd drop-in for the ordering workaround, but its reported persistence across reboots and updates is specific to the author’s environment. If you have already created a drop-in and need to undo it, remove the file you created and reload the user systemd manager; the exact file path depends on that procedure. Do not delete unrelated systemd configuration files.
If Chrome 154 may be involved
Record the browser version and whether the failure began after running it. KDE discussion suggests temporarily avoiding Chrome while investigating, but the available material does not verify an official Chromium or Ubuntu resolution. Do not treat a browser association as proof of cause or delete arbitrary cache directories as a blanket fix.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Do not confuse a crash with post-upgrade layout problems
Kubuntu’s release notes separately warn that a major Plasma 6.6, Qt 6.10, and Frameworks transition may leave some upgraders with theme, panel, layout, or widget problems. For those configuration symptoms—not a fontconfig crash—the official release notes suggest reapplying the global theme first. They describe a clean ~/.config as a last resort after backing up. Do not use configuration resets as a substitute for checking a crash trace.
When the evidence does not match
If the trace lacks fontconfig frames, the crashing process is not plasmashell, or the package and browser clues do not fit, none of the reported fontconfig remedies is established as relevant. Keep the crash details and session warnings, identify the component named in the backtrace, and investigate that failure path instead of repeating cache deletions. The reports available here do not establish a confirmed official resolution or current package status for the suspected fontconfig incidents.
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