The info about the non-free version of ffmpeg is not up to date. Using Reaper 5.7, I have to copy all dll files to the plugin folder. It is not enough to copy the libraries avcodec, avformat, avutil and swscale.
Also, I can report that the latest version of ffmpeg compatible with Reaper is v3.4.1 (
https://ffmpeg.zeranoe.com/builds/wi...n64-shared.zip). Using the nightly from Februrary 1st (
https://ffmpeg.zeranoe.com/builds/wi...n64-shared.zip) does not work as Reaper will simply ignore these libraries.