Terrible advice from flowreen91.
The gist you linked to fixes a different issue.
I fixed it on my Arch machine somehow but I don't really know what of the things I did, fixed it.
First I used the
vapoursynth check-envcommand and I could see that the VSSCRIPT_PATH was not set.
So I executed
vapoursynth get-vsscript and set the output of it as the VSSCRIPT_PATH environment variable.
It didn't seem to fix the issue even after restarting SVP, but after a computer restart it worked.