Download page for windows builds links to non-working ffmpeg binaries See download page https://kdenlive.org/download/ The download link for ffmpeg points to https://ffmpeg.zeranoe.com/builds/win64/shared/ffmpeg-20171217-387ee1d-win64-shared.zip which isn't working for some reason. Please point the link to working ffmpeg distro. I found this forum: https://forum.kde.org/viewtopic.php?f=265&t=143241&p=385141#p385141 and suggested ffmpeg https://ffmpeg.zeranoe.com/builds/win64/shared/ffmpeg-3.4.1-win64-shared.zip did the trick for me.
Should be fixed with Kdenlive version 18.04 FFMPEG is included