No login screen after upgrade to kubuntu 16.04

[Jan W]

After upgrading kubuntu from 15.10 to 16.04, my system no longer boots to the login screen. There is a lot of flickering and the kubuntu logo is sometimes shown, sometimes a black screen, finally resulting in a black screen.
I am able to switch to a terminal and login to a text prompt and see the following line from dmesg:

sddm[3968]: segfault at 0 ip 00007f31f9a776c3 sp 00007fff6b1c9b60 error 4 in libQt5Core.so.5.5.1

I discovered that removing the nvidia driver package resolves the situation and makes it possible to login to the desktop again. However, that makes it impossible for me to use my Nvidia graphics device.

I have a laptop with an Intel skylake CPU (i5-6200U) and an Nvidia Geforce 940M graphics card.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: kubuntu-desktop 1.338
ProcVersionSignature: Ubuntu 4.4.0-21.37-generic 4.4.6
Uname: Linux 4.4.0-21-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: KDE
Date: Sat Apr 30 20:59:40 2016
InstallationDate: Installed on 2016-02-03 (87 days ago)
InstallationMedia: Error: [Errno 13] Permission denied: '/var/log/installer/media-info'
SourcePackage: kubuntu-meta
UpgradeStatus: No upgrade log present (probably fresh install)

[Jan W (ubuntu-kiekerjan)]

More googling suggests this has to do with using bumblebee to use the nvidia and intel GPUs. I used the following blog to install the nvidia drivers.

http://lenovolinux.blogspot.nl/2016/05/bumblebee-on-lenovo-t440p-nvidia-gt.html

I can now use either GPU at my disposal:

$ glxgears -info
Running synchronized to the vertical refresh. The framerate should be
approximately the same as the monitor refresh rate.
GL_RENDERER = Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2)
GL_VERSION = 3.0 Mesa 11.2.0
GL_VENDOR = Intel Open Source Technology Center

$ optirun glxgears -info
GL_RENDERER = GeForce 940M/PCIe/SSE2
GL_VERSION = 4.5.0 NVIDIA 361.42
GL_VENDOR = NVIDIA Corporation