Can't render a video in KDenlive 0.9 on Ubuntu Studio 12.04 64bit

Hello everyone, I am experiencing some serious and common problems everytime I try to render a video.
The fact is that no matter which format I choose, the program creates a 0 byte file and not the video I want.
I also tried to remove the facedetection files in the folder /usr/lib/frei0r-1 but I had not been able to find them, so the problem must be something else.
Can someone help me, please?