Liferea Version 1.8.0, OS: Arch Linux (up to date)
If I create an empty folder I can't drag and drop feeds into it. As soon as there is at least one feed in the folder I can drag and drop the others.
If you drop a feed into a folder you see (because of the black line) if you're dropping into the folder or into the root. There is NO black line to drop into the folder if the folder is empty (has that "(Emtpy)" label in it). So it might be a gtk issue, but that's nothing more than a wild guess.
If you need more information please ask.