Kubuntu (Plasma 5) 16.04 GLX/rendering errors -- possibly a packaging issue?

Hi folks,

I’m trying to run Plasma 5 alongside a “stock” Ubuntu 16.04 environment with Nvidia-364 installed from this PPA (https://launchpad.net/~graphics-drivers/+archive/ubuntu/ppa) which I understand to be officially supported-ish.

However, every time I try to boot into Plasma 5, the desktop is all black (though the DE appears to be actually running properly), and running an application like systemsettings5 (even running it from under a unity-desktop session), I get output that looks like this, and black boxes over most graphical objects:

Error loading plugin "kcm_lookandfeel" "The shared library was not found." 
Plugin search paths are ("/usr/lib/x86_64-linux-gnu/qt5/plugins", "/usr/bin") 
The environment variable QT_PLUGIN_PATH might be not correctly set
QSGTextureAtlas: texture atlas allocation failed, code=500
Xlib:  extension "NV-GLX" missing on display ":0".

Any idea what could be going on here? The QT_PLUGIN_PATH variable indeed seems not to be set, but the kcm_lookandfeel plugin exists in the relevant path, and I’m assuming those are not actually the issue (because they should be configured exactly as shipped).

This is probably not an upstream Nvidia issue unless recent versions of 364 are supposed to setting up that NV-GLX extension and aren’t doing it properly, and it’s not either a breakage with that PPA’s debs or the way Plasma 5 looks for the extension. All GPU acceleration works fine under Gtk desktops in this environment and there are no rendering issues. Any ideas?