If you still have this problem, try downloading the portable version of Vapoursynth R40 from here: https://github.com/vapoursynth/vapoursy … ble-R40.7z
Extract the file vsscript.dll from that archive into your MPV folder (C:\Program Files (x86)\SVP 4\mpv64) overwriting the file that is already there. MPV is refusing to work with the newer versions of that file for some reason.