Alright, I did it. ;)
My Kubuntu doesn't load the login screen or the graphical interface anymore but stops booting when it prints "Starting TiMidity++ ALSA midi emulation... [ OK ]".
However, I can't tell exactly which action has caused this:
Before some parts of my workspace have crashed and restarted, and afterwards the workspace was rather slow.
Yet before I restarted the system, I've also changed the owner of the folders /var and /etc to root (stupid idea? ^^) in order to fix another problem (message was sth like "... is owned by uid 1000, should be 0").
So I was wondering now, if there's a way to fix this problem without a complete reinstallation.
Thanks in advance and just ask if you think you need to know some more things. :)