I use Ubuntu 9.04 and Kdenlive 0.7.4...
As soon as I add a clip from the Project Tree to the "real project section"(I don't know how it's called...), Kdenlive crashes instantly and I get that message:

"A Fatal Error Occurred
The application Kdenlive (kdenlive) crashed and caused the signal 11 (SIGSEGV)."

That is probably an install issue. Unfortunately a lot of users run into this because they have an old version of MLT installed, or are trying to run several versions (Build Wizard, packages, self compiled,...).

First, make sure you don't have older MLT pachages like 0.3.8.
Then, you can try to delete your $HOME/.kde/share/config/kdenliverc file, and restart Kdenlive. The config wizard will tell you if he can find a correct MLT...

Good luck

It's version 0.4.2, isn't that the latest already? :/

Yes, MLT 0.4.2 is the latest version, but depending on how you installed, you may still have some older MLT or MLT++ packages or libraries on the system...

Somehow it works now, but many effects in the effects-stack are gone... p_p

That's how I installed it:
sudo apt-get install melt && sudo apt-get install kdenlive