had the same issue here showing only invalid files, but was able to fix it, at least for me
This is the original ffmpeg path entry in the file /c/webroot/AirVideo/readynas.properties
path.ffmpeg = ./AirVideo/ffmpeg
This file is available in this directory, but nevertheless the ffmpeg plugin installed the ffmpeg to /usr/local/bin/ffmpeg
So I changed the path entry to
path.ffmpeg = /usr/local/bin/ffmpeg
and after a restart of the plugin I'm happy
Just for the records:
-rwxr-xr-x 1 root root 7973448 2010-10-29 22:35 /c/webroot/AirVideo/ffmpeg
-rwxr-xr-x 1 root staff 8302976 2011-05-20 09:26 /usr/local/bin/ffmpeg



