So I'm using the latest kdenlive release in Ubuntu, and, no matter the format, have been unable to successfully render a video of more than 45 seconds. For anything larger than that, it gets into the 50-70% Range, and then crashes, giving this response:

Rendering of /home/matt/Videos/Free Vermont/June 1-3 2010 - Kirby Trip/Day 2/Pete Garritano Intro.avi crashed
Unable to parse option value "dv_ntsc": undefined constant or missing (
Last message repeated 1 times [h264 @ 0x89f63a0]B picture before any references, skipping [h264 @ 0x89f63a0]decode_slice_header error [h264 @ 0x89f63a0]no frame! [h264 @ 0x89f63a0]B picture before any references, skipping [h264 @ 0x89f63a0]decode_slice_header error [h264 @ 0x89f63a0]no frame!
[h264 @ 0x8c1e710]B picture before any references, skipping [h264 @ 0x8c1e710]decode_slice_header error [h264 @ 0x8c1e710]no frame!
[h264 @ 0x8c1e710]B picture before any references, skipping [h264 @ 0x8c1e710]decode_slice_header error [h264 @ 0x8c1e710]no frame!
[h264 @ 0x8d30d80]B picture before any references, skipping
[h264 @ 0x8d30d80]decode_slice_header error
[h264 @ 0x8d30d80]no frame!
[h264 @ 0x8d30d80]B picture before any references, skipping
[h264 @ 0x8d30d80]decode_slice_header error
[h264 @ 0x8d30d80]no frame!

Help?

Please provide more information about your system and someone might be able to help you out.
Such as..

-Which Ubuntu Lucid? 32 or 64 bit?
-where did you get kdenlive/mlt/ffmpeg from and what version numbers?
-Where did the source files come from? What kind of camera and format? How captured?
-Any effects or other things that might be going on?
-What happens if you try a few different input clips?
-Have you tried other output formats like DVD or mpeg4, etc..

-Not sure how to tell (I'm a fairly linux illiterate user :-/).
-I got kdenlive from the ubuntu software center, ver. 0.7.7.1
-Kodak Zx1 HD, which exports the video in .MOV format.
-The effects are pretty much dissolve transitions and volume changes.
-Short projects render, longer (>~45 seconds) crash as described above.
-All the output formats have the same issue.

A couple thoughts. Maybe one of the developers will find these forum posts, but you should probably file a bug. See some info here: http://kdenlive.org/bug-reports

This maybe something that you can workaround if you find out why it is crashing, but I really don't know. I've found kdenlive to be very stable with my files, but I'm sure there can be issues with so many different formats out there. I noticed your camera isn't on the supported camcorder list, but that doesn't mean it won't work either. Looks like it does a common format, but who knows.

Some things to try and note:

-Can you play each of the clips from a terminal using "ffplay FILENAME" ?
-Can you play through the entire video and watch it in the "project monitor" before rendering in kdenlive?
-Can you make another version without any effects?
-You could try changing other rendering options like changing it so it rescales to something like 320x240 which will greatly speed up your trials.
-What did you choose for "project settings"? Try starting over and creating a new project with project settings of "HD 720p 30fps" or some other common settings.

I'm just guessing here...
Hope this helps.

On my Archlinux rendering with a dv profile still works.
Seems to be a problem with Ubuntu as more Ubuntu users reported this.
As a temporary fix try a different format (MPEG-4 for example).