Quote Originally Posted by ZStefan View Post
Can somebody knowledgeable look at this log file and tell where the fault is: in Qt5, KDE, Nvidia driver, version mismatch, somewhere else? Maybe a new version of the currently-defective software will arrive soon.
This crash is in glib.
*Might* be caused by the nvidia driver, wouldn't be the first time that it causes strange crashes/hangs in glib because of bugs in the signal handling.

More details about your backtrace:
Some kded module requests root permissions via kauth.
kauth in turn uses polkit to verify if this is allowed (and ask for the root password if necessary), which then crashes.

So to fix those crashes, find out *which* kded module is causing this, by disabling them one by one.
I would speculate it is the backlight control that requires root permissions (which would again point to an nvidia driver related problem), so try to disable the powermanagement module first (in Configure Desktop->Startup and Shutdown->Background Services) and see if it "fixes" the crashes.