When I was trying to use ffmpeg to convert flv files into other format, it gave the ``symbol lookup error'' message. After searching solutions via Google, I found installing the latest version by compiling from the source code worked for me.
So, just remove the installed ffmpeg in Ubuntu, then go to ffmpeg website to download the latest source code. Follow the install procedure (./configure, make, and make install) then ffmpeg works well.
ffmpeg -i input_file.flv output_file.mpg