Ubuntu 16.04, KDE 5.5.5.

You cannot move windows around nor resize them. Changing the focus and setting one windows to the front still works.

Also copy&paste does not work anymore.

This condition sometimes happens (maybe once in a month or so).

This very much describes the problem and provides a working solution for me:
http://lemire.me/blog/2005/09/28/strange-kde-bug-cant-resize-or-move-windows/windows

Note that the report is from 2005! And still people commenting on it (2017) that this works for them.

Basically, what (temporarily) solved the problem for me was:

killall kwin_x11
kwrapper5 kwin_x11 --replace