Anytime the player lists something as “Unknown”, it means it cannot read the ID3 tag of the file(s). Either the tag is in the wrong format, or other discrepancies within the fields of the tag. I’m curious though; I assume these are the same files that displayed correctly on your other players? The Clip+ is not that different, and that information should be displayed the same.
As far as I know, .pla playlists works ok on the Clip+. I only use .m3u playlists myself, so can’t verify that, but I know there are many posts here on the forum on the subject and I don’t recall any complaining that .pla playlists don’t work.
One thing though . . . if you convert your .pla playlists to .m3u, do not put that playlist file in the Playlist folder. That won’t work. Only .pla playlists go into the Playlist folder; .m3u playlists must be in the same [parent] folder as the music files themselves (i.e. Music).
Hope this helps. 
[Edit]: One more thing . . . I don’t believe playlists will work with files in the Audiobook folder. I think they have to be in the Music folder. You might try moving your audiobook folders/files into the Music folder and see if that might not help. 
