kwin --replace does, unfortunately, not solve the problem (it does reload but the menu is still not accessible). I found another option:
killall -9 plasmashell
QSG_RENDER_LOOP=basic /usr/bin/plasmashell &
Not sure if the QSG_RENDER_LOOP variable is required, but the plasmashell crash is (temporarily) resolved. Have to run this script every 5-10 minutes which is OK for now but not on the long run, of course.