KDE Plasma 6.4 and Screen Locker error

Just a quick post in case this helps someone.

My PC (Fedora 42) has just updated to KDE Plasma 6.4.

Immediately, every time I locked the session (Super key + L), an error appeared on the screen

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+F1), log in to your account and execute the command:

loginctl unlock-session 2

Then log out of the virtual session by pressing Ctrl+D, and switch back to the running session (Ctrl+Alt+F2).

Should you have forgotten the instructions, you can get back to this screen by pressing Ctrl+Alt+F2.

Firstly, for some reason, tty1 wouldn't work, so I needed to use Ctrl+Alt+F3 to get to tty3.

But I didn't want to be doing that every time my session locked.

Searching the web I found various bug reports (none recent) that sounded similar. There was nothing in the KDE bug tracker to match the error (that was opened since the Plasma 6.4 release). I went down various blind alleys that didn't help: changing my wallpaper from picture of the day to a fixed image, looking into the error file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/lockscreen/LockOsd.qml:10:1: "../osd": no such directory, and the error "ml: The backend got an unknown wallpaper provider type. Both those errors were popping up in journalctl at exactly the moment I locked my session and got the error message.

In the end, something someone posted somewhere prompted me to change the desktop theme. I simply opened the Settings app, and clicked to choose Breeze Dark.

Now my sessions lock and unlock without a hitch.

Update 24 hours later: Take 2

Not so fast!

Before applying a theme freshly, this error would happen every time I locked my session.

Now, I can mostly lock and unlock fine, but sometimes (maybe 1 in 10?) I still get the error screen.

The one message I'm seeing in my journalctl that seems relevant is this one:

The backend got an unknown wallpaper provider type. The wallpaper will now fall back to the default. Please check your wallpaper configuration!

That led me to this page on /Fedora reddit.

That pointed me to two things

1. Someone claimed this is a known bug that should be patched within a week. Wait and see
2. If the problem is caused by lock screen wallpaper, set that to a new wallpaper manually. That's System Settings > Screen Locking > Change Appearance. I've done that. Wait and see

Take 3

Another 24 hours, and the problem is still happening. It seems the lockscreen wallpaper may have been causing errors, but was not the problem.

I found two threads on the Fedora forum and the KDE forum. They suggest that the upgrade of KDE Plasma may not be teh cause, but that qtwayland6 updated at the same time and that is at fault. So, following the tips there, I have downgraded that.

sudo dnf downgrade qt6-qtwayland-0:6.9.1-1.fc42.x86_64

(You need to have the fedora-updates-archive.repo enabled for dnf to find the older version.)

Because the fault is intermittent, I now need to wait another 24 hours before I can be sure it has been solved. I'll report back.

Another 24 hours later

I’ve now had 24 hours without a hitch. That said, no sooner did I come here to update this, but a comment has popped up from someone who found the downgrade did not solve it for them. So maybe the mystery rumbles on.

I see that a tagged release of qt6-qtwayland is being reviewed by Fedora prior to release. That will be 6.9.1-3, and will be identical to 6.9.1-1 but with a higher minor release number so that people can upgrade to undo the damage of going to 6.9.1-2.