it should be like this
mpv input -o output --no-audio --no-sub --no-sub-auto --input-ipc-server=mpvencodepipe --vf=vapoursynth:/some-path/ffff.py:4:16 --ofps 60.000 --of=matroska --ovc=h264_nvenc --ovcopts=b=11055051,rc=vbr_hq,preset=slow,profile=high,threads=16ovc=h264_nvenc
You may try to run the exact mpv command line from the log in console directly, it will probably give a more verbose output.