I didn't at the time but now I did and it worked. Why?!
Anyway, I tested somewhere else on the F:\ drive and it worked as well.
But then I moved it in the same scenario as where the folders above are (there's a "!" in the path which is used in the fodler name to keep them on top of other folders) and... it didn't work anymore. Made double-sure the paths were fine... nope... Rebooted, clicked my button and tried again. Nope.
I'm entirely lost.
My NAS is a Synology DS1821+ NAS using the latest OS version. If that helps...