There has been also reported a bug in debian bts for plasma-workspace:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=801739

Quote from comment
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=801739#25:
> But in my case the crash appears in
> plasmashell after login from kdm. Apparently that is run
> in the context of the user. Adding kdm to video does
> not help, but adding the user does.

I've also added my user to the video group in /etc/group and login via kdm and lightdm into plasma works now.

sddm is also working when adding additionally sddm to the video group in /etc/group

If not doing so I get following Error:

Application: sddm-greeter (sddm-greeter), signal: Aborted
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7ff1977ee8c0 (LWP 2837))]

Thread 2 (Thread 0x7ff183fff700 (LWP 2839)):
#0  0x00007ff1953e052d in poll () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007ff19246b1dc in g_main_context_iterate (priority=2147483647, n_fds=1, fds=0x7ff17c003070, timeout=<optimized out>, context=0x7ff17c0009b0) at /build/glib2.0-BMd9vh/glib2.0-2.46.0/./glib/gmain.c:4135
#2  0x00007ff19246b1dc in g_main_context_iterate (context=context@entry=0x7ff17c0009b0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at /build/glib2.0-BMd9vh/glib2.0-2.46.0/./glib/gmain.c:3835
#3  0x00007ff19246b2ec in g_main_context_iteration (context=0x7ff17c0009b0, may_block=1) at /build/glib2.0-BMd9vh/glib2.0-2.46.0/./glib/gmain.c:3901
#4  0x00007ff196216e4b in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#5  0x00007ff1961bd2ca in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#6  0x00007ff195fdb374 in QThread::exec() () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#7  0x00007ff196d45055 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#8  0x00007ff195fe025e in  () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#9  0x00007ff193d910a4 in start_thread (arg=0x7ff183fff700) at pthread_create.c:309
#10 0x00007ff1953e906d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Thread 1 (Thread 0x7ff1977ee8c0 (LWP 2837)):
[KCrash Handler]
#4  0x00007ff195338107 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#5  0x00007ff1953394e8 in __GI_abort () at abort.c:89
#6  0x00007ff195fcdc28 in  () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#7  0x00007ff196ff5ee1 in  () at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#8  0x00007ff196ffdee4 in  () at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#9  0x00007ff196ffe30b in  () at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#10 0x00007ff1964ea7f5 in QWindow::event(QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#11 0x00007ff19702e9a3 in QQuickWindow::event(QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#12 0x00007ff1961bf959 in QCoreApplication::notify(QObject*, QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#13 0x00007ff1961bfa8b in QCoreApplication::notifyInternal(QObject*, QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#14 0x00007ff1964e3366 in QGuiApplicationPrivate::processExposeEvent(QWindowSystemInterfacePrivate::ExposeEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#15 0x00007ff1964e40ad in QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#16 0x00007ff1964c9628 in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#17 0x00007ff19774daf0 in  () at /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqxcb.so
#18 0x00007ff19246afe7 in g_main_context_dispatch (context=0x7ff184001710) at /build/glib2.0-BMd9vh/glib2.0-2.46.0/./glib/gmain.c:3154
#19 0x00007ff19246afe7 in g_main_context_dispatch (context=context@entry=0x7ff184001710) at /build/glib2.0-BMd9vh/glib2.0-2.46.0/./glib/gmain.c:3769
#20 0x00007ff19246b240 in g_main_context_iterate (context=context@entry=0x7ff184001710, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at /build/glib2.0-BMd9vh/glib2.0-2.46.0/./glib/gmain.c:3840
#21 0x00007ff19246b2ec in g_main_context_iteration (context=0x7ff184001710, may_block=1) at /build/glib2.0-BMd9vh/glib2.0-2.46.0/./glib/gmain.c:3901
#22 0x00007ff196216e2f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#23 0x00007ff1961bd2ca in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#24 0x00007ff1961c4e3c in QCoreApplication::exec() () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#25 0x0000000000414548 in main ()

Thanks, that's practically expected if we can't access openGL.

Could you tell me what error we're getting when we print out an error here: 
#4 0x00007ff195338107 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56

(i.e run 
DISPLAY=:0 plasmashell and tell me the last line or paste ~/.xsession-errors after a failed logging in)

*** This bug has been marked as a duplicate of bug 345563 ***

Here is the full output of DISPLAY=:0 plasmashell, hope that contains the requested information

stefan@neptun:~$ DISPLAY=:0 plasmashell
kscreen: launcherDataAvailable: "org.kde.KScreen.Backend.XRandR"
kscreen: Primary output changed from KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" ) to KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" )
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
last screen is < 0 so putting containment on screen  0
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
file:///usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/FolderView.qml:366:13: QML GridView: Binding loop detected for property "overflowing"
kscreen: Launcher finished with exit code 1 , status 0
kscreen: Service for requested backend already running
kscreen: Primary output changed from KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" ) to KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" )
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
Failed to create OpenGL context for format QSurfaceFormat(version 2.0, options QFlags(), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior 2, swapInterval 1, profile  0) 
Application::crashHandler() called with signal 6; recent crashes: 1
/usr/bin/plasmashell  --crashes 1 &
KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = plasmashell path = /usr/bin pid = 6946
KCrash: Arguments: /usr/bin/plasmashell 
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi from kdeinit
sock_file=/run/user/1000/kdeinit5__0
kscreen: launcherDataAvailable: "org.kde.KScreen.Backend.XRandR"
kscreen: Primary output changed from KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" ) to KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" )
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
last screen is < 0 so putting containment on screen  0
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
file:///usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/FolderView.qml:366:13: QML GridView: Binding loop detected for property "overflowing"
kscreen: Launcher finished with exit code 1 , status 0
kscreen: Service for requested backend already running
kscreen: Primary output changed from KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" ) to KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" )
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
Failed to create OpenGL context for format QSurfaceFormat(version 2.0, options QFlags(), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior 2, swapInterval 1, profile  0) 
Application::crashHandler() called with signal 6; recent crashes: 2
/usr/bin/plasmashell  --crashes 2 &

[2]+  Angehalten              DISPLAY=:0 plasmashell
stefan@neptun:~$ KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = plasmashell path = /usr/bin pid = 6958
KCrash: Arguments: /usr/bin/plasmashell --crashes 1 
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi from kdeinit
sock_file=/run/user/1000/kdeinit5__0
kscreen: launcherDataAvailable: "org.kde.KScreen.Backend.XRandR"
kscreen: Primary output changed from KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" ) to KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" )
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
unversioned plugin detected, may result in instability
last screen is < 0 so putting containment on screen  0
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
file:///usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/FolderView.qml:366:13: QML GridView: Binding loop detected for property "overflowing"
kscreen: Launcher finished with exit code 1 , status 0
kscreen: Service for requested backend already running
kscreen: Primary output changed from KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" ) to KScreen::Output(Id: 640 , Name: "DVI-I-3" ) ( "DVI-I-3" )
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
No metadata file in the package, expected it at: "/usr/share/wallpapers/Evening/contents/images//metadata.desktop"
Failed to create OpenGL context for format QSurfaceFormat(version 2.0, options QFlags(), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior 2, swapInterval 1, profile  0) 
Application::crashHandler() called with signal 6; recent crashes: 3
/usr/bin/plasmashell  --crashes 3 &
Too many crashes in short order or respawning disabled, not restarting automatically.
KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = plasmashell path = /usr/bin pid = 6978
KCrash: Arguments: /usr/bin/plasmashell --crashes 2 
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi from kdeinit
sock_file=/run/user/1000/kdeinit5__0