Session switching broken

After I switch between sessions (switch to new user session via Kstart->leave->switch_user, but also when switching to text console via Ctrl+Alt+F1), and after I try to go back to the original session, the screen is completely garbled, i.e. the screen is filled with blinking black&white triangles of all shapes and sizes, sometimes there is a functional mouse cursor.

Here are the things I tried in order to narrow down the issue:

- In system settings (Display and Monitor ---> Compositor ---> Rendering Backend) I switched from OpenGL 3.1 to OpenGL 2.0
- I changed display drivers from nvidia-346 to nvidia-346-updates
- I disabled screen lock
- I disabled virtual desktops

Neither of the above actions improved matters.

Here are the things which I also noticed:

- Restarting sddm (sudo service sddm stop) restores a functioning login screen
- Typing Ctrl+Alt+F1 on the login screen, and then Alt+F7 to go back gives me a working login screen
- Killing kwin_x11 kills the window decorations, but restores a working screen image.
- I frequently saw kwin running at 100% system load, which I vaguely remember to be an old issue with kwin, it may be unrelated to my current woes
- nothing of value shows up in dmesg

The above points tell me, that this is not a graphics driver issue, but one in kwin, but obviously I may be wrong. If so, please let me know. I am more than willing to test debug versions of kwin, self compiled if necessary, or whatever it takes to track down the root issue.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: kwin 4:5.2.2a-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-28.30-generic 3.19.8-ckt5
Uname: Linux 3.19.0-28-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.17.2-0ubuntu1.5
Architecture: amd64
CurrentDesktop: KDE
Date: Fri Sep 25 14:23:18 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2012-09-25 (1094 days ago)
InstallationMedia: Kubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120423)
SourcePackage: kwin
UpgradeStatus: Upgraded to vivid on 2015-09-16 (9 days ago)