I have been working with both FFmpeg and avconv for about 2 years now focusing on ubuntu based systems. Some of the tasks I had worked are improving, researching and testing many codecs like libx264 (h264 codec), libvpx, libvorbis, libfdk-aac, libvo_aacenc, etc., using all sort of video clips and I am currently working on a FFmpeg instalation script for ubuntu based systems to match Zeranoe builds but by having the option to create a static or a shared build directly from each codec repository.
Also, I replicated your issue and fixed it in my laptop. Just be sure to have admin rights to install in your ubuntu :)