Hello everybody,

After completing all the available updates I noticed that suddenly some of the applications, such as krunner wouldn't work anymore and others could not be terminated anymore,so, I had to use the shutdown shell command to terminate Kubuntu. When I try to start up the system, the booting procedure stops at the bootsplash. Watching the booting procedure in the command line mode does not show any errors. After the line saying something like ...user-session... the screen is cleared. Entering the command line mode again, I can login as usual. However, if I try to strart PlasmaShell, I get the following error:

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~/kstart5 plasmashell
qt.qpa.xcb: could not connect to display
qt.qta.plugin: could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because not Qt platform plugin could be initialized. reinstalling the app may fix this problem
Available platform plugins are: wayland-org.kde.kwin.qpa, eglfs, ...
fish: 'kstart5 plasmashell' terminated by signal SIGABRT (Abbruch)

Reinstalling xcb did not fix anything.

Using startx I get a very basic Gnome Shell. However, most of the desktop applications and design features are not working here, too. Also, the display configuration of the system settings is not available.

Does anybody have a suggestion, how I can get my Plasma Desktop up and running again? Any help would be highly appreciated.

Which Kubuntu version, which exact Plasma version?

Thanks very much for your reply. Sorry it was not easy for me to find out all the versions, as plasmashell --version' does not work because of the same error. However, I am running

Kubuntu 19.10
plasma-desktop 5.18.0
Qt 5.12.4
KDE Frameworks 5.67.0
Kf5-config 1.0

BTW running the command kf5-config --version says it couldn't detrermine $DISPLAY. So, I set the value to 0 but this did not change anything and after rebooting the the value has gone again. If I don't reboot and run kstart5 plasmashell instead, I get the above error message again just with hte display number added to the first line.

did you try running an update again on the command line? Maybe you miss some packages

Yes, I did, also using the --fix missing and --fix-broken options but the packages seem to be ok. The strange thing was that the problems already started, when I was running PlasmaShell for the last time. Suddenly, some of the applications did not react any more and after a reboot, I got that error message.

Is this a genuine installation or a VM one? I just upgraded to plasma 5.18.0 myself and don't have any problems.

Hi Mamarok,

it is a genuine installation including all updates available from the main and backdoor repos. I am also using quite a number of snap anf flatpak apps but everything used to work fine anutil a couple of days before. It is hard to imagine that oine of the package updates would crush such a basic function like connecting to the display.

I could try to use a different platform plugin, if I knew how to change it.

"backdoor" repos, nice typo

I doubt the backports are to blame, I tend to suspect flatpack to cause problems. Do you really need those?
I have never even activated snap, and try to keep away from flatpack as much as possible, mixing source is rarely a good idea.

Yes, Iagree and I can actually hardly imagine that oane of the snap or flatpak apps is resposbile foe the problem, as they run kind of sandboxed in their cntainers.

However, I also posted the problem in other forums and didn't receive a single reply there. So, obviously the issue I ran into seems to be a very special one and it looks like it's time for another fresh install of Kubuntu or maybe Neon this time.

Thanks for your support, anyway.

Just be aware that Neon is not a distribution but a Plasma and KDE showcase, so if you are also using non-KDE applications you better choose Kubuntu instead.

After the number of problems increased, I decided to run a fresh install and moved to Neon User Edition, which solved all the issues I had. Neon requires a bit of additional installation work but on the other hand, it does not install applications I do not need. It works great.

Thanks Mamarok for your support.

I get similar error messages. (I had a crash after installing KWave. No recovery so far. I removed KWave, but the damage is done.)
I only have command line.

The old startx and kstart5 are now history
There are no sure fire replacements to restart plasmashell and kwin.

Any ideas out there now that Wayland is in and Qt involved?

I copied documents, videos, etc. from my Neon partition to MS Win 10 folders as a backup, then nuked and paved with Neon 5.18.1 and am now restoring as much as I can that got wiped out given how Spartan Neon is.

I my case what fixed it was doing:

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sudo apt full-upgrade libx11-*

This will remove and reinstall all libx11 components.