I've started with kdenlive two days ago and I've created a short (5 Minutes) video. Rendering to DV/AVI with DV PAL 4:3 finished after some time with no errors. After rendering, I can import this rendered file to a project and playback this file within kdenlive.

Surprisingly, I cannot playback the rendered videos directly from HDD with Banshee, VLC or Xine.

What does ffplay or mediainfo tell you about the file?

I've found the solution. Previous bevore rendering, kdenlive crashed and I started it new and then I've got the mentioned problem. After logging out from desktop and restarting kdenlive, all works fine.

Problem solved.