So I got the usual generic answers from Slate and Kilohearts (make sure there's no leftovers from previous installations then reinstall etc...) and it didn't help, BUT I've found a solution that's working in my situation, can't say if it will work for others too:
I've moved all the DLLs from the folder "Kilohearts" to the parent folder "Vstplugins" and Studio One recognized the plugins without an issue...
That's really weird but it works for me!
I hope it will work for you too :)