The result: after restart and KDE relogin I ended up with a black screen. Only two windows (firefox and dolphin) were remaining on the black desktop. I was able to switch betwen these both windows but not able to open new ones (e.g. with Alt-F2).
After login with IceWM, I switched off the composite manager and made a relogin to KDE. Now KDE runs, but of course without any desktop effects. When I switch on the desktop effect I can directly have a look on a black desktop :-(.
Today’s update brought in some qtlibs and that solved the problem for me. Although I did have an odd problem where yesterday’s update got rid of my nvidia driver. I just put it back.