X11 segfaulting since update (Arch Linux)

Do have debuginfod active ? Try turning it on and get the stack trace again, it should be more informative.

/usr/lib/xorg/modules/extensions/libvnc.so

That’s not in KDE software, neither is Xorg.
Are you using vnc, remote screen ?

It looks like:

You might want to consider the workaround: Random Log Outs in X11 - #19 by meven