What worked for me is creating a new empty playlist in the folder - like it was explained here - and than expand the folder with the little arrow in the navigation menue. After that it was possible to drag and drop a playlist in/"under" the folder like you would drag and drop a playlist to sort it in the navigation menue.
Drag and drop the playlist "into" the folder or "over" the folder did not work.
There appears to be an issue between desktop and mobile in regards to Playlist folders:
I drag and drop playlists and it looks fine on desktop, but when I access on my mobile device, completely different playlists appear in each folder. Not consistent with what is showing on desktop at the same time.
And how do I remove playlists from a folder, or delete a folder, without deleting the playlists in it?
I'm having issues with drag-and drop, as it appears to work at first, but when I click the arrow to hide the contents of the folder, the playlists remain visible, regardless of the position of the arrow on the folder.