ProPainter Vapoursynth filter

[Dan64]

Now it works for me too. 
Strange, must be some cosmic rays 

I do agree with you. 
I still don't understand why is working. 
It seems something related to "png" format used to save mask.

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To create the "working" mask, I used:

1) Corel Paint Shop Pro
2) Created a new image of size 352x288 (resolution 300 pixel/inch)
3) Image characteristics: Raster Background, Color Black (depth: RGB 8 bits/channel)
4) Added the mask
5) Saved the file as PNG: Noninterlaced, Optimized palette

Dan