On Saturday August 18 2018 09:15:08 huw wrote: >I just press ctrl-alt-backspace twice to fix this...I think it restarts >the X server or something?
Yes, that's the standard shortcut for killing the X11 server, effectively logging you off and falling back to the login screen. A thought: did you get the new login manager when you installed the Plasma5 environment? It's not officially a KDE app so there's a change it wasn't installed.
