This is mostly caused by ffmpeg-php not working properly. Since ffmpeg-php use ffmpeg, this can caused by ffmpeg or ffmpeg-php.

So try reinstalling ffmpeg and ffmpeg-php latest versions. Download ffmpeg from SVN. Also ffmpeg-php latest version is 0.5.1

http://docs.buyscripts.in/Server_Setup

If you are using vshare 2.6, you can select mplayer in admin area for thumbnail creation, in that case, you don't need ffmpeg-php installed.


Site Settings > Miscellaneous Settings > Create Thumbnail with