I'm using kdenlive 0.7.8_0 on Mac OSX 10.6.4 and in the last day I attempted to see if I could get better performance of kdenlive by activating ffmpeg-devel (I was using ffmpeg before) as well as updating [via Macports] py26-numpy and gcc44 (I believe with the atlas variant I can't remember exactly) and immediately afterwards I went to my working project and attempted to add more clips only to be prompted with the following message:
"Clip /Users/username/Movies/Video Clips/filename.xyzformat is missing or invalid. Remove it from project?"
If I choose no it loads the clip into my project tree but does not display a thumbnail image like it normally does. Also if I decide to drag it into the timeline the program crashes.
I have gone back uninstalled ffmpeg-devel, activated ffmpeg, uninstalled kdenlive (just the single prot kdenlive and left all of the dependencies alone) and reinstalled kdenlive and still get the error.
Does anyone have any advice on how I could possible solve this without having to rebuild all of kdenlive including its dependencies? I've done that way too many times now haha.

If you change ffmpeg to ffmpeg-devel, you must rebuild everything using ffmpeg. In this case, you must rebuild mlt: sudo port -n upgrade --force mlt.

Ah, it works! Thank you, you're so unbelievably helpful.