ksmserver 100% CPU when select shutdown or logoff

This bug only happens when nvidia drivers are used.
Occasionally (but not always) when I click shutdown or log off, the system doesn't exit, ksmserver goes to 100% CPU and the system becomes unresponsive (or very very slow to open a new window). This is a fresh install of Kubuntu 15.10.
$ kdeinit5 --version
Qt: 5.4.2
KDE: 5.15.0
$ ksmserver -v
Qt: Session management error: networkIdsList argument is NULL
ksmserver 0.4
$ journalctl -t ksmserver
 -- No entries --
$ dpkg -l | grep nvidia
rc nvidia-352 352.63-0ubuntu0.15.10.1 amd64 NVIDIA binary driver - version 352.63
rc nvidia-352-updates 352.63-0ubuntu0.15.10.1 amd64 NVIDIA binary driver - version 352.63
rc nvidia-opencl-icd-352 352.63-0ubuntu0.15.10.1 amd64 NVIDIA OpenCL ICD
ii nvidia-opencl-icd-352-updates 352.63-0ubuntu0.15.10.1 amd64 NVIDIA OpenCL ICD
ri nvidia-prime 0.8.1 amd64 Tools to enable NVIDIA's Prime
ri nvidia-settings 352.21-0ubuntu1 amd64 Tool for configuring the NVIDIA graphics driver

The driver manager in system settings offers 3 graphics drivers;
- nvidia 352 (recommended)
- nvidia 352-updates
- x.org nouveau display driver

I have tried with all three drivers (with a reboot between changes) and the freeze-on-shutdown symptoms occur with each of the nvidia drivers, but not (so far) with the x.org driver.

I have two machines here running Kubuntu 15.10. One has nvidia graphics card and it has this problem. The other machine has Intel graphics, and has not exhibited any symptoms like this.