This report cannot be handled without further information. Please run "gdb --args kwin --replace" from the command line, and when it crashes, use "bt" to obtain a backtrace.

If the panel also crashes, it could be a lower level problem, such as with video drivers. Please check, which other components besides KDE you updated.

you neither get a "kwin just crashed" dialog?

smells like OOM - depending on how ubuntu is configured, there will be a notion in demesg; otherwise just watch out for memory usage.

in any case, a better description and ideally a backtrace is inevitable.

PS: if this happens "after i updated the system", that's pretty common with ubuntu.
suspicion is that they truncate files on installation ("use cp instead of install")

"you neither get a "kwin just crashed" dialog?"

Normally no, but it seems to have given me one this time:

Application: KWin (kwin), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f5a6f8357c0 (LWP 2255))]

Thread 3 (Thread 0x7f5a4f4af700 (LWP 2271)):
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
#1  0x00007f5a6e20006b in ?? () from /usr/lib/x86_64-linux-gnu/libQtScript.so.4
#2  0x00007f5a6e2000a9 in ?? () from /usr/lib/x86_64-linux-gnu/libQtScript.so.4
#3  0x00007f5a66adcf6e in start_thread (arg=0x7f5a4f4af700) at pthread_create.c:311
#4  0x00007f5a6f00e9cd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:113

Thread 2 (Thread 0x7f5a4da41700 (LWP 2280)):
#0  0x00007f5a6f006de3 in select () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007f5a6933337b in qt_safe_select (nfds=17, fdread=0x7f5a44000ac8, fdwrite=0x7f5a44000d60, fdexcept=0x7f5a44000ff8, orig_timeout=0x0) at kernel/qcore_unix.cpp:83
#2  0x00007f5a69338904 in QEventDispatcherUNIXPrivate::doSelect (this=this@entry=0x7f5a44000910, flags=..., timeout=0x0) at kernel/qeventdispatcher_unix.cpp:223
#3  0x00007f5a69338d02 in QEventDispatcherUNIX::processEvents (this=0x7f5a440008f0, flags=...) at kernel/qeventdispatcher_unix.cpp:937
#4  0x00007f5a693075ef in QEventLoop::processEvents (this=this@entry=0x7f5a4da40d70, flags=...) at kernel/qeventloop.cpp:149
#5  0x00007f5a693078e5 in QEventLoop::exec (this=this@entry=0x7f5a4da40d70, flags=...) at kernel/qeventloop.cpp:204
#6  0x00007f5a6920688f in QThread::exec (this=this@entry=0x2323080) at thread/qthread.cpp:542
#7  0x00007f5a692e8d13 in QInotifyFileSystemWatcherEngine::run (this=0x2323080) at io/qfilesystemwatcher_inotify.cpp:265
#8  0x00007f5a69208f2f in QThreadPrivate::start (arg=0x2323080) at thread/qthread_unix.cpp:338
#9  0x00007f5a66adcf6e in start_thread (arg=0x7f5a4da41700) at pthread_create.c:311
#10 0x00007f5a6f00e9cd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:113

Thread 1 (Thread 0x7f5a6f8357c0 (LWP 2255)):
[KCrash Handler]
#6  0x00007f5a68839400 in QRegion::intersect (this=0x20903a8, r=...) at painting/qregion.cpp:4164
#7  0x00007f5a68839599 in intersected (r=..., this=<optimized out>) at ../../include/QtGui/../../src/gui/painting/qregion.h:113
#8  QRegion::operator& (this=<optimized out>, r=...) at painting/qregion.cpp:509
#9  0x00007f5a6f3a3633 in KWin::Scene::Window::clientShape (this=this@entry=0x2090370) at ../../kwin/scene.cpp:672
#10 0x00007f5a6f3a528a in KWin::Scene::paintSimpleScreen (this=this@entry=0x2a366d0, orig_mask=orig_mask@entry=8, region=...) at ../../kwin/scene.cpp:280
#11 0x00007f5a6f3a3155 in KWin::Scene::finalPaintScreen (this=0x2a366d0, mask=mask@entry=8, region=..., data=...) at ../../kwin/scene.cpp:184
#12 0x00007f5a6f3c4cdd in KWin::EffectsHandlerImpl::paintScreen (this=0x2a4a2c0, mask=mask@entry=8, region=..., data=...) at ../../kwin/effects.cpp:374
#13 0x00007f5a6ead8cbf in KWin::Effect::paintScreen (this=this@entry=0x2d3be20, mask=mask@entry=8, region=..., data=...) at ../../../kwin/libkwineffects/kwineffects.cpp:522
#14 0x00007f5a6f3c4c9a in KWin::EffectsHandlerImpl::paintScreen (this=0x2a4a2c0, mask=8, region=..., data=...) at ../../kwin/effects.cpp:371
#15 0x00007f5a6f3a5548 in KWin::Scene::paintScreen (this=this@entry=0x2a366d0, mask=mask@entry=0x7fff01cfd5ec, region=region@entry=0x7fff01cfd6a0) at ../../kwin/scene.cpp:141
#16 0x00007f5a6f3b7d37 in KWin::SceneOpenGL::paint (this=0x2a366d0, damage=..., toplevels=...) at ../../kwin/scene_opengl.cpp:360
#17 0x00007f5a6f39d4d3 in KWin::Compositor::performCompositing (this=0x20756c0) at ../../kwin/composite.cpp:618
#18 0x00007f5a69320be1 in QObject::event (this=0x20756c0, e=<optimized out>) at kernel/qobject.cpp:1156
#19 0x00007f5a686a5dfc in QApplicationPrivate::notify_helper (this=this@entry=0x1cfe7f0, receiver=receiver@entry=0x20756c0, e=e@entry=0x7fff01cfda90) at kernel/qapplication.cpp:4567
#20 0x00007f5a686ac470 in QApplication::notify (this=this@entry=0x7fff01cfdf60, receiver=receiver@entry=0x20756c0, e=e@entry=0x7fff01cfda90) at kernel/qapplication.cpp:4353
#21 0x00007f5a6dc54a6a in KApplication::notify (this=0x7fff01cfdf60, receiver=0x20756c0, event=0x7fff01cfda90) at ../../kdeui/kernel/kapplication.cpp:311
#22 0x00007f5a693088bd in QCoreApplication::notifyInternal (this=0x7fff01cfdf60, receiver=0x20756c0, event=0x7fff01cfda90) at kernel/qcoreapplication.cpp:946
#23 0x00007f5a69338403 in sendEvent (event=<optimized out>, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#24 QTimerInfoList::activateTimers (this=0x1cff338) at kernel/qeventdispatcher_unix.cpp:622
#25 0x00007f5a69338480 in QEventDispatcherUNIX::activateTimers (this=this@entry=0x1c39ad0) at kernel/qeventdispatcher_unix.cpp:879
#26 0x00007f5a69338d90 in QEventDispatcherUNIX::processEvents (this=this@entry=0x1c39ad0, flags=...) at kernel/qeventdispatcher_unix.cpp:941
#27 0x00007f5a687482f6 in QEventDispatcherX11::processEvents (this=0x1c39ad0, flags=...) at kernel/qeventdispatcher_x11.cpp:152
#28 0x00007f5a693075ef in QEventLoop::processEvents (this=this@entry=0x7fff01cfddd0, flags=...) at kernel/qeventloop.cpp:149
#29 0x00007f5a693078e5 in QEventLoop::exec (this=this@entry=0x7fff01cfddd0, flags=...) at kernel/qeventloop.cpp:204
#30 0x00007f5a6930ce5b in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1218
#31 0x00007f5a686a434c in QApplication::exec () at kernel/qapplication.cpp:3828
#32 0x00007f5a6f357956 in kdemain (argc=1, argv=0x7fff01cfe0a8) at ../../kwin/main.cpp:597
#33 0x00007f5a6ef35de5 in __libc_start_main (main=0x4006d0 <main(int, char**)>, argc=1, ubp_av=0x7fff01cfe0a8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fff01cfe098) at libc-start.c:260
#34 0x00000000004006fe in _start ()

are you still experiencing these crashes?

It was a kubuntu problem, not kde