I use Studio One 3.5 x64 version on Windows PC.
I use x64 version of my plugin.
Yes I have 3rd party VST support for my SO.
If I copy my .dll in 'C:\Program Files\Common Files\VST directory' it is not visible by SO.
If I copy the same .dll in some random folder on 'C:\' lets name it "Dupa", so the path will be: 'C:\Dupa' it will work fine.
Right now I have only these 2 folders added in 'Locations' pane in my Studio One.
I can't see why it is happening. I do not want to have separate folder for just this one VST.