Think I've worked out what the trouble was with #Kubuntu 17.10.1 and #Kmail & #KOrganizer

If you apt install the #libgl1 it uninstalls #libgl1-mesa-glx packages and then Kmail and KOrganizer can't initialise GLX which they both need.

So guess that's what happened the other day. Something uninstalled the *-mesa-glx stuff.

Discovered this when I was to trying to get LibreOffice to run with OpenGL support on.