KDE Neon Plasma session crash after updates: diagnostic guide

Problem: Plasma logs in then drops back to SDDM

After entering credentials at SDDM on KDE Neon you get a black screen, a desktop with no panel or wallpaper, or a crash loop back to the greeter with repeated plasmashell crashed notifications. This typically appears right after an update was applied. Neon ships an Ubuntu LTS base for kernel, drivers and core libraries, and KDE Neon repositories for Plasma, Qt and Frameworks. A broken session is most often a version mismatch between those two layers, not a hardware failure.

Recognizable condition

• Login succeeds briefly then returns to SDDM
• Plasmashell restarts continuously
• Wayland session fails but X11 works, or vice versa
• Symptoms start immediately after Discover or apt update

Cause to diagnostic signal

Ordered checks

1. Isolate session type

Log out to SDDM and try both Plasma on Wayland and Plasma on X11. A failure limited to Wayland points to graphics/driver, a failure on both points to packages or config.

2. Check disk space before package work

Run in a TTY, e.g. Ctrl+Alt+F3, as your user:

df -h / /boot

Risk: low. Expected check: several gigabytes free on / and at least a few hundred megabytes on /boot. If full, clean old kernels via apt autoremove before proceeding.

3. Check dpkg health

Run as root or with sudo:

sudo dpkg --audit

Then:

sudo apt --fix-broken install

Expected check: no packages reported as unconfigured or broken. Do not run Discover, apt and pkcon concurrently; finish one frontend to avoid dpkg lock conflicts.

4. Verify repository origin for Plasma and Qt

Run:

apt policy plasma-workspace

Expected check: installed and candidate versions are from neon.* repositories. If a PPA or Ubuntu archive provides a higher candidate, Neon packages can be shadowed by incompatible builds.

5. Complete pending upgrades

Run:

sudo apt update

Then dry run:

sudo apt full-upgrade --dry-run

If upgrades remain, complete them in one transaction:

sudo apt full-upgrade

Partial upgrades are the leading cause of mismatched Qt/Plasma libraries. Completing the set is the first fix to try.

6. Inspect logs for session and driver errors

From a TTY as your user:

journalctl -b -u sddm --no-pager | head -n 200

And for the user session:

journalctl --user -b --no-pager | grep -i plasmashell

For X11 also check ~/.xsession-errors. Look for driver or KMS errors, not specific error codes.

7. Test Plasma cache and config isolation

Rename, do not delete, user Plasma data to test isolation:

mv ~/.cache ~/.cache.bak

Then:

mv ~/.config/plasma* ~/.config/plasma*.bak

Restart SDDM:

sudo systemctl restart sddm

Log in. If the session is stable, the issue was stale caches or config surviving the update. A wholesale reset of ~/.config discards all customization.

Fixes tied to findings

• Pending upgrades: finish apt full-upgrade and finish the Discover queue, then reboot.
• Broken dpkg: apt --fix-broken install, then apt full-upgrade.
• Repository conflict: remove offending PPAs, run apt update and apt full-upgrade. ppa-purge and manual downgrades can cascade; back up or snapshot first.
• NVIDIA Wayland failure: ensure driver modules match the running kernel and DRM modesetting is enabled. Kernel and driver changes only take effect after a reboot. If Wayland remains unstable, use X11 session as a temporary workaround.
• Cache/config corruption: restore needed files from the .bak copies instead of keeping the reset.

Verification

After each change restart SDDM and log in fresh, not just restart plasmashell. Confirm the fix survives a reboot and at least one further update cycle.

Run again:

apt update && apt full-upgrade --dry-run && dpkg --audit

And confirm apt policy for plasma-workspace and Qt packages still resolves to neon.* repositories.

Escalation criteria

Escalate when:

• apt policy shows mixed origins and apt full-upgrade cannot resolve without removing core Neon packages
• Journal shows persistent kernel module load failures after matching driver and kernel versions
• Session fails for all users and on both Wayland and X11 even with clean caches and completed upgrades

Limitations: Neon Ubuntu base version and Plasma release change over time, so exact package names, session options and driver requirements must be confirmed against current Neon documentation. This guide is conservative and intended for human review.