The upgrade of LC was interrupted because I forgot I had protected some customisation in the LUA files with chattr +i (I was unable to accomplish it with the custom.lua). After removing the attribute I installed again and was left with a broken LC. The frontend screens seem to miss the style sheets, after log-in and probably more.
Is it safe to attempt a apt-get --reinstall ? Since MySQL backup was not made either (see my other bug report in the German section), I am not sure any more what I can do without damaging it more. I meanwhile did make dump but I don't know if that is still OK.