• caused by libmvtools like I first assumed, since the latest dev only comes with the older libmvtools.
  
• that vsViewer doesn't find the portable Vapoursynth (that comes with Hybrid), since the latest dev comes with a vsviewer.config and properly displays the version info.
  
• any other filtering, since you checked that Vapoursynth Script View does not show that anything aside from the source filter:
  
  # loading plugins
core.std.LoadPlugin(path="<Hybrid>/64bit/vsfilters/SourceFilter/LSmashSource/LSMASHSource.dll")
# Import scripts
import validate
  gets loaded.
  
• a general Windows 10 issue since it does work fine on my updated Windows 10 laptop.
  

• the source filter,
  
• some general Vapoursynth issue, or
  
• something interfering (could be a background service)
  

1. create a test.vpy file with the following content:
   
   # Imports
import vapoursynth as vs
# getting Vapoursynth core
import sys
import os
core = vs.core
# loading plugins
core.std.LoadPlugin(path="C:/Program Files/Hybrid/64bit/vsfilters/SourceFilter/LSmashSource/LSMASHSource.dll")
# Source: 'S:\Nasties-2398-IN\Trailer 28 MinutesHB.mp4'
# Current color space: YUV420P8, bit depth: 8, resolution: 720x576, frame rate: 23.976fps, scanorder: progressive, yuv luminance scale: limited, matrix: 470bg, transfer: bt.709, primaries: bt.601 ntsc, format: AVC
# Loading S:\Nasties-2398-IN\Trailer 28 MinutesHB.mp4 using LibavSMASHSource
clip = core.lsmas.LibavSMASHSource(source="S:/Nasties-2398-IN/Trailer 28 MinutesHB.mp4")
clip.set_output()
2. open that file in vsViewer and press F5
   
3. open a Terminal Window inside "C:/Program Files/Hybrid/64bit/Vapoursynth" and then call:
   
   VSPipe.exe "PATH TO the test.vpy" NUL -c y4m --progress
4. open a Terminal Window inside "C:/Program Files/Hybrid/64bit/Vapoursynth" and then call:
   
   VSPipe.exe "PATH TO the test.vpy" --info
5. Does it make a difference if you open the TaskManager and check that under processes no other 'Apps' are running when using Hybrid?
   

Quote:open a Terminal Window inside "C:/Program Files/Hybrid/64bit/Vapoursynth"