kwin_x11 goes crazy after waking up from suspend-to-RAM

After waking up my laptop computer (HP EliteBook 8560w) from suspend-to-RAM, I very often but not always (perhaps around 80%) cannot unlock the screen, because parts of the windows (often triangles) of the programs that are currently running (and basically should be "behind" rather than in front of the lock screen) flicker around so that I cannot enter my password. I found a solution that mostly but not always (perhaps around 60-70%) works: use Ctrl-Alt-F6 to switch to a text console, log in, execute "pkill kwin_x11", and use Alt-F7 to switch back to the graphical console; then I can unlock the screen, because the flickering windows have disappeared and the lock screen is in the foreground; after unlocking the screen, the window decorations have disappeared and I cannot switch between windows etc, but I can use Alt-F2 and execute "kwin_x11", which brings back the window decorations and usually everything works again as it should.

Sometimes after killing kwin_x11 (perhaps around 20-30%), a program window (which is no longer flickering) is in front of the lock screen so that I cannot unlock the screen. If I kill this program from the text console, the window disappears but it is replaced by a black rectangle, which covers the lock screen so that I still cannot unlock the screen. In these cases, it seems that the only thing I can do is to kill Xorg and re-login or to restart the computer (from the text console).

I sometimes put the computer into or take it out of a docking station with an external monitor while the computer is suspended but the problem also occurs (perhaps a little less often) if the computer is not put into or taken out of the docking station while suspended.

Please let me know if you have any questions.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: kwin 4:5.2.2a-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-18.18-generic 3.19.6
Uname: Linux 3.19.0-18-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
CurrentDesktop: KDE
Date: Tue Jun 2 10:44:01 2015
ExecutablePath: /usr/bin/kwin_x11
InstallationDate: Installed on 2012-02-12 (1206 days ago)
InstallationMedia: Kubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
SourcePackage: kwin
UpgradeStatus: Upgraded to vivid on 2015-05-08 (25 days ago)
XsessionErrors:
 (process:2415): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
 (process:3923): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
 (process:5543): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
 (deja-dup-monitor:1820): GLib-CRITICAL **: Source ID 50 was not found when attempting to remove it
 (deja-dup-monitor:1820): GLib-CRITICAL **: Source ID 50 was not found when attempting to remove it