It's a problem I've seen for a long time, which I remember with plasma 5 since when there are certain updates, the "ksmserver-logout-greeter" (in this case after an NVIDIA driver update, from 435 to 440) goes into crash and you can't turn off the KDE menu: "exit> turn off", but you have to force it from the CASE PC button or the "systemctl poweroff" terminal.
The crash brought me information, I hope it's useful.

Kubuntu 20.04 (but the problem as mentioned is in any version.)
Framework: 5.64.0-0ubuntu1 amd64


Application: ksmserver-logout-greeter (ksmserver-logout-greeter), signal: Aborted
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f8a6cc3b840 (LWP 31045))]

Thread 4 (Thread 0x7f8a69916700 (LWP 31049)):
#0  0x00007f8a70900c2f in __GI___poll (fds=0x7f8a580025e0, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x00007f8a6eeada6e in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007f8a6eeadba3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007f8a70ea7943 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#4  0x00007f8a70e4e8bb in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#5  0x00007f8a70c877f5 in QThread::exec() () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#6  0x00007f8a72238479 in ?? () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#7  0x00007f8a70c88a42 in ?? () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#8  0x00007f8a6f728669 in start_thread (arg=<optimized out>) at pthread_create.c:479
#9  0x00007f8a7090d323 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 3 (Thread 0x7f8a6b08e700 (LWP 31047)):
#0  __GI___libc_read (nbytes=16, buf=0x7f8a6b08db20, fd=6) at ../sysdeps/unix/sysv/linux/read.c:26
#1  __GI___libc_read (fd=6, buf=0x7f8a6b08db20, nbytes=16) at ../sysdeps/unix/sysv/linux/read.c:24
#2  0x00007f8a6eef5d8f in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007f8a6eead5be in g_main_context_check () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#4  0x00007f8a6eeada12 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#5  0x00007f8a6eeadba3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#6  0x00007f8a70ea7943 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#7  0x00007f8a70e4e8bb in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#8  0x00007f8a70c877f5 in QThread::exec() () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#9  0x00007f8a71711efa in ?? () from /lib/x86_64-linux-gnu/libQt5DBus.so.5
#10 0x00007f8a70c88a42 in ?? () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#11 0x00007f8a6f728669 in start_thread (arg=<optimized out>) at pthread_create.c:479
#12 0x00007f8a7090d323 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 2 (Thread 0x7f8a6bdbd700 (LWP 31046)):
#0  0x00007f8a70900c2f in __GI___poll (fds=0x7f8a6bdbcc68, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x00007f8a704a4917 in ?? () from /lib/x86_64-linux-gnu/libxcb.so.1
#2  0x00007f8a704a653a in xcb_wait_for_event () from /lib/x86_64-linux-gnu/libxcb.so.1
#3  0x00007f8a6c58a1c8 in ?? () from /lib/x86_64-linux-gnu/libQt5XcbQpa.so.5
#4  0x00007f8a70c88a42 in ?? () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#5  0x00007f8a6f728669 in start_thread (arg=<optimized out>) at pthread_create.c:479
#6  0x00007f8a7090d323 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 1 (Thread 0x7f8a6cc3b840 (LWP 31045)):
[KCrash Handler]
#6  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#7  0x00007f8a70810899 in __GI_abort () at abort.c:79
#8  0x00007f8a70c4faaf in QMessageLogger::fatal(char const*, ...) const () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#9  0x00007f8a725b9065 in QSGRenderLoop::handleContextCreationFailure(QQuickWindow*, bool) () from /lib/x86_64-linux-gnu/libQt5Quick.so.5
#10 0x00007f8a725ba157 in ?? () from /lib/x86_64-linux-gnu/libQt5Quick.so.5
#11 0x00007f8a725baa1d in ?? () from /lib/x86_64-linux-gnu/libQt5Quick.so.5
#12 0x00007f8a712426dd in QWindow::event(QEvent*) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#13 0x00007f8a7263c149 in QQuickWindow::event(QEvent*) () from /lib/x86_64-linux-gnu/libQt5Quick.so.5
#14 0x0000565007ff9916 in ?? ()
#15 0x00007f8a71909a66 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16 0x00007f8a719130e0 in QApplication::notify(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#17 0x00007f8a70e4fd1a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#18 0x00007f8a71238c26 in QGuiApplicationPrivate::processExposeEvent(QWindowSystemInterfacePrivate::ExposeEvent*) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#19 0x00007f8a71238e54 in QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent*) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#20 0x00007f8a712132ab in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#21 0x00007f8a6c58b25e in ?? () from /lib/x86_64-linux-gnu/libQt5XcbQpa.so.5
#22 0x00007f8a6eead87d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#23 0x00007f8a6eeadb00 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#24 0x00007f8a6eeadba3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#25 0x00007f8a70ea7925 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#26 0x00007f8a70e4e8bb in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#27 0x00007f8a70e56626 in QCoreApplication::exec() () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#28 0x0000565007ff57cc in ?? ()
#29 0x00007f8a708121e3 in __libc_start_main (main=0x565007ff5540, argc=1, argv=0x7ffc8d5df6c8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffc8d5df6b8) at ../csu/libc-start.c:308
#30 0x0000565007ff591e in _start ()
[Inferior 1 (process 31045) detached]