I have been hit by #643726 again.
I was happily working on a terminal while an apt upgrade was running on
another, and suddenly kbuildsycoca4 started vomiting on my command line
because I had run okular once on that terminal last tuesday.
Among the possible workarounds, one can run kdebugdialog from package
kde-runtime and check the "Disable all debug output" button.
This is the output of kdebugdialog when run on a terminal after enabling
"Disable all debug output":
$ kdebugdialog QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave. QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
I see it as debugging output turned into an art performance, and I have an
appreciation for this kind of things. kdebugdialog, my hat's off to you.