Application: kwin_x11 (5.8.6)
 (Compiled from sources)
Qt Version: 5.6.2
Frameworks Version: 5.29.0
Operating System: Linux 3.17.7-gentoo x86_64
Distribution: "Gentoo Base System release 2.3"

-- Information about the crash:
- What I was doing when the application crashed:

During the login.
Now I have the DE but w/o the window manager...

The crash can be reproduced every time.

-- Backtrace:
Application: KWin (kwin_x11), signal: Segmentation fault
Using host libthread_db library "/lib64/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f57be1f27c0 (LWP 6208))]

Thread 4 (Thread 0x7f57bd02d700 (LWP 6211)):
#0  0x00007f57d276520d in poll () from /lib64/libc.so.6
#1  0x00007f57d18c38f2 in _xcb_conn_wait () from /usr/lib64/libxcb.so.1
#2  0x00007f57d18c56df in xcb_wait_for_event () from /usr/lib64/libxcb.so.1
#3  0x00007f57bdf00e09 in QXcbEventReader::run() () from /usr/lib64/libQt5XcbQpa.so.5
#4  0x00007f57d04acb3c in QThreadPrivate::start(void*) () from /usr/lib64/libQt5Core.so.5
#5  0x00007f57d2a27454 in start_thread () from /lib64/libpthread.so.0
#6  0x00007f57d276e28d in clone () from /lib64/libc.so.6

Thread 3 (Thread 0x7f57b7359700 (LWP 6213)):
#0  0x00007f57d2766f13 in select () from /lib64/libc.so.6
#1  0x00007f57d0692f2f in qt_safe_select(int, fd_set*, fd_set*, fd_set*, timespec const*) () from /usr/lib64/libQt5Core.so.5
#2  0x00007f57d06945de in QEventDispatcherUNIXPrivate::doSelect(QFlags<QEventLoop::ProcessEventsFlag>, timespec*) () from /usr/lib64/libQt5Core.so.5
#3  0x00007f57d0694af2 in QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQt5Core.so.5
#4  0x00007f57d0646fda in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQt5Core.so.5
#5  0x00007f57d04a8724 in QThread::exec() () from /usr/lib64/libQt5Core.so.5
#6  0x00007f57d2f73475 in QDBusConnectionManager::run() () from /usr/lib64/libQt5DBus.so.5
#7  0x00007f57d04acb3c in QThreadPrivate::start(void*) () from /usr/lib64/libQt5Core.so.5
#8  0x00007f57d2a27454 in start_thread () from /lib64/libpthread.so.0
#9  0x00007f57d276e28d in clone () from /lib64/libc.so.6

Thread 2 (Thread 0x7f57b54dc700 (LWP 6225)):
#0  0x00007f57d2a2d3d8 in pthread_cond_timedwait () from /lib64/libpthread.so.0
#1  0x00007f57d04ad168 in QWaitCondition::wait(QMutex*, unsigned long) () from /usr/lib64/libQt5Core.so.5
#2  0x00007f57d04a9c02 in QThreadPoolThread::run() () from /usr/lib64/libQt5Core.so.5
#3  0x00007f57d04acb3c in QThreadPrivate::start(void*) () from /usr/lib64/libQt5Core.so.5
#4  0x00007f57d2a27454 in start_thread () from /lib64/libpthread.so.0
#5  0x00007f57d276e28d in clone () from /lib64/libc.so.6

Thread 1 (Thread 0x7f57be1f27c0 (LWP 6208)):
[KCrash Handler]
#6  0x00007f57b4b77610 in vtable for Oxygen::Decoration () from /usr/lib64/qt5/plugins/org.kde.kdecoration2/oxygendecoration.so
#7  0x00007f57d0650b39 in QMetaObject::cast(QObject const*) const () from /usr/lib64/libQt5Core.so.5
#8  0x00007f57b493e421 in Oxygen::Button::create(KDecoration2::DecorationButtonType, KDecoration2::Decoration*, QObject*) () from /usr/lib64/qt5/plugins/org.kde.kdecoration2/oxygendecoration.so
#9  0x00007f57cef6714f in KDecoration2::DecorationButtonGroup::DecorationButtonGroup(KDecoration2::DecorationButtonGroup::Position, KDecoration2::Decoration*, std::function<KDecoration2::DecorationButton* (KDecoration2::DecorationButtonType, KDecoration2::Decoration*, QObject*)>) () from /usr/lib64/libkdecorations2.so.5
#10 0x00007f57b49481c0 in Oxygen::Decoration::createButtons() () from /usr/lib64/qt5/plugins/org.kde.kdecoration2/oxygendecoration.so
#11 0x00007f57b4949c04 in Oxygen::Decoration::init() () from /usr/lib64/qt5/plugins/org.kde.kdecoration2/oxygendecoration.so
#12 0x00007f57d2389768 in KWin::Decoration::DecorationBridge::createDecoration(KWin::AbstractClient*) () from /usr/lib64/libkwin.so.5
#13 0x00007f57d2261f71 in KWin::Client::createDecoration(QRect const&) () from /usr/lib64/libkwin.so.5
#14 0x00007f57d2262bdb in KWin::Client::updateDecoration(bool, bool) () from /usr/lib64/libkwin.so.5
#15 0x00007f57d22c5c07 in KWin::Client::manage(unsigned int, bool) () from /usr/lib64/libkwin.so.5
#16 0x00007f57d2242504 in KWin::Workspace::createClient(unsigned int, bool) () from /usr/lib64/libkwin.so.5
#17 0x00007f57d2249284 in KWin::Workspace::init() () from /usr/lib64/libkwin.so.5
#18 0x00007f57d2249f17 in KWin::Workspace::Workspace(QString const&) () from /usr/lib64/libkwin.so.5
#19 0x00007f57d22a4133 in KWin::Application::createWorkspace() () from /usr/lib64/libkwin.so.5
#20 0x00007f57d2c4302e in QtPrivate::QFunctorSlotObject<KWin::ApplicationX11::performStartup()::{lambda()#2}::operator()() const::{lambda()#1}, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) () from /usr/lib64/libkdeinit5_kwin_x11.so
#21 0x00007f57d066f83e in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib64/libQt5Core.so.5
#22 0x00007f57d2c4338b in QtPrivate::QFunctorSlotObject<KWin::ApplicationX11::performStartup()::{lambda()#2}, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) () from /usr/lib64/libkdeinit5_kwin_x11.so
#23 0x00007f57d066f83e in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib64/libQt5Core.so.5
#24 0x00007f57d13f9dab in KSelectionOwner::Private::claimSucceeded() () from /usr/lib64/libKF5WindowSystem.so.5
#25 0x00007f57d067053b in QObject::event(QEvent*) () from /usr/lib64/libQt5Core.so.5
#26 0x00007f57d0ecaf9c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib64/libQt5Widgets.so.5
#27 0x00007f57d0ed04d6 in QApplication::notify(QObject*, QEvent*) () from /usr/lib64/libQt5Widgets.so.5
#28 0x00007f57d064816a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /usr/lib64/libQt5Core.so.5
#29 0x00007f57d0695f83 in QTimerInfoList::activateTimers() () from /usr/lib64/libQt5Core.so.5
#30 0x00007f57d0694ad4 in QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQt5Core.so.5
#31 0x00007f57bdf7538d in QUnixEventDispatcherQPA::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQt5XcbQpa.so.5
#32 0x00007f57d0646fda in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQt5Core.so.5
#33 0x00007f57d064e95c in QCoreApplication::exec() () from /usr/lib64/libQt5Core.so.5
#34 0x00007f57d2c4527b in kdemain () from /usr/lib64/libkdeinit5_kwin_x11.so
#35 0x00007f57d26a7790 in __libc_start_main () from /lib64/libc.so.6
#36 0x0000000000400809 in _start ()

Reported using DrKonqi