Recent install of Kubuntu 15.04. Updated video driver to the latest NVIDIA for my Geforce 840M in an HP envy laptop. Now on boot I get a pop-up saying sddm greeter has crashed on about 3 out of 4 boot attempts. The pop-up provides choices to restart application or just continue. Restart produces all black screen, continue produces all white screen. In either case, nothing can be doe except power down using the power button.

Reproducible: Sometimes

Steps to Reproduce:
1. Install latest NVIDIA driver using the "driver manager" application.
2. Reboot.

Actual Results:  
sddm greeter crash. blank screen. Unable to do anything.

Expected Results:  
Normal boot to login screen.

stacktrace:

Application: sddm-greeter (sddm-greeter), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f58a66dc780 (LWP 957))]

Thread 3 (Thread 0x7f5899303700 (LWP 958)):
#0  0x00007f58a477a8dd in poll () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007f58a6441b72 in ?? () from /usr/lib/x86_64-linux-gnu/libxcb.so.1
#2  0x00007f58a644364f in xcb_wait_for_event () from /usr/lib/x86_64-linux-gnu/libxcb.so.1
#3  0x00007f589c348099 in QXcbEventReader::run (this=0x1649bf0) at qxcbconnection.cpp:1105
#4  0x00007f58a5011b0e in QThreadPrivate::start (arg=0x1649bf0) at thread/qthread_unix.cpp:337
#5  0x00007f58a3d566aa in start_thread (arg=0x7f5899303700) at pthread_create.c:333
#6  0x00007f58a4785eed in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

Thread 2 (Thread 0x7f5893fff700 (LWP 1216)):
#0  0x00007f58a477649d in read () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007f58a1bddf45 in ?? () from /usr/lib/nvidia-346-updates/tls/libnvidia-tls.so.346.59
#2  0x00007f58a25382f0 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007f58a24f496c in g_main_context_check () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#4  0x00007f58a24f4e60 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#5  0x00007f58a24f4fcc in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#6  0x00007f58a52a8c6c in QEventDispatcherGlib::processEvents (this=0x7f588c0008c0, flags=...) at kernel/qeventdispatcher_glib.cpp:418
#7  0x00007f58a524d3e2 in QEventLoop::exec (this=this@entry=0x7f5893ffee20, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#8  0x00007f58a500cb44 in QThread::exec (this=<optimized out>) at thread/qthread.cpp:503
#9  0x00007f58a5cd7f65 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#10 0x00007f58a5011b0e in QThreadPrivate::start (arg=0x18ac810) at thread/qthread_unix.cpp:337
#11 0x00007f58a3d566aa in start_thread (arg=0x7f5893fff700) at pthread_create.c:333
#12 0x00007f58a4785eed in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

Thread 1 (Thread 0x7f58a66dc780 (LWP 957)):
[KCrash Handler]
#6  QScreen::handle (this=0x0) at kernel/qscreen.cpp:85
#7  0x00007f589c35c545 in QXcbWindow::QXcbWindow (this=0x1f6e700, window=0x1666260) at qxcbwindow.cpp:260
#8  0x00007f589c3495c1 in QXcbIntegration::createPlatformWindow (this=<optimized out>, window=0x1666260) at qxcbintegration.cpp:194
#9  0x00007f58a55a1786 in QWindowPrivate::create (this=0x166bd40, recursive=recursive@entry=false) at kernel/qwindow.cpp:395
#10 0x00007f58a55a197b in QWindow::create (this=this@entry=0x1666260) at kernel/qwindow.cpp:539
#11 0x00007f58a55a4678 in QWindow::setVisible (this=0x1666260, visible=<optimized out>) at kernel/qwindow.cpp:475
#12 0x0000000000421bf4 in SDDM::GreeterApp::show() ()
#13 0x00007f58a52809c9 in QMetaObject::activate (sender=0x1822b30, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=0x0) at kernel/qobject.cpp:3716
#14 0x0000000000428a80 in SDDM::ScreenModel::initScreens(bool) ()
#15 0x0000000000428c67 in SDDM::ScreenModel::onScreenAdded(QScreen*) ()
#16 0x00007f58a52809c9 in QMetaObject::activate (sender=0x7ffd60118750, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x7ffd60117f70) at kernel/qobject.cpp:3716
#17 0x00007f58a5281057 in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7f58a59e88c0 <QGuiApplication::staticMetaObject>, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x7ffd60117f70) at kernel/qobject.cpp:3
582
#18 0x00007f58a5592c52 in QGuiApplication::screenAdded (this=<optimized out>, _t1=0x164c240) at .moc/moc_qguiapplication.cpp:321
#19 0x00007f58a5589569 in QPlatformIntegration::screenAdded (this=<optimized out>, ps=<optimized out>) at kernel/qplatformintegration.cpp:439
#20 0x00007f589c34644f in QXcbConnection::updateScreens (this=this@entry=0x163c130) at qxcbconnection.cpp:303
#21 0x00007f589c34727d in QXcbConnection::handleXcbEvent (this=this@entry=0x163c130, event=event@entry=0x7f5894004010) at qxcbconnection.cpp:961
#22 0x00007f589c3484bb in QXcbConnection::processXcbEvents (this=0x163c130) at qxcbconnection.cpp:1303
#23 0x00007f58a528173a in QObject::event (this=0x163c130, e=<optimized out>) at kernel/qobject.cpp:1245
#24 0x00007f58a524faf5 in QCoreApplication::notify (this=<optimized out>, receiver=<optimized out>, event=<optimized out>) at kernel/qcoreapplication.cpp:997
#25 0x00007f58a524fc2b in QCoreApplication::notifyInternal (this=0x7ffd60118750, receiver=0x163c130, event=event@entry=0x7f5894005e00) at kernel/qcoreapplication.cpp:935
#26 0x00007f58a5251c9b in sendEvent (event=0x7f5894005e00, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:228
#27 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0, data=0x16152d0) at kernel/qcoreapplication.cpp:1552
#28 0x00007f58a5252298 in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1410
#29 0x00007f58a52a8843 in postEventSourceDispatch (s=0x166c7c0) at kernel/qeventdispatcher_glib.cpp:271
#30 0x00007f58a24f4c3d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#31 0x00007f58a24f4f20 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#32 0x00007f58a24f4fcc in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#33 0x00007f58a52a8c57 in QEventDispatcherGlib::processEvents (this=0x1664060, flags=...) at kernel/qeventdispatcher_glib.cpp:418
#34 0x00007f58a524d3e2 in QEventLoop::exec (this=this@entry=0x7ffd601186c0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#35 0x00007f58a525502c in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1188
#36 0x00000000004111b7 in main ()