Ah, okay, thanks, that's helpful. Unfortunately, I'm writing a JRiver driver for use in an RTI IDE using embedded JavaScript, and the networking library they provide does not give me much control over the connection - all I can do easily is "get" a URL and process the XML response. So I'll go back to working with passing in the Token returned by Authenticate.
In any case, you'd mentioned earlier that the Playlist/Delete failure probably means that the playlist to be deleted could not be found. Is there any other cause you can think of? My driver exercises more than one function that takes an existing playlist as a parameter, and I haven't had any problem with the others.
Also, the reason I focused on authentication as a possible problem is that Playlist/Delete appears to require it, unlike the other functions I'm calling. Is it indeed true that this particular function does require authentication?