Bring that playlist files locally, play that and see if the problem persists. It'll tell us if the problem is network related or not. Let us know what happens.
Does the "waiting" message come up at almost the same time during playing the last track?
Can you remove the last track from playlist thus making the second last one the last track? When the "waiting" message comes up, can you now drag in the removed last track in the playlist and see what happens?
Also try by dragging in the removed last track in the playlist just before the "waiting message comes up. Let us know what happens.
Navigate to your network drive (containing playlist files) using a file explorer and keep that window open. When the waiting message comes up, from that file explorer, perform some activity on the network drive. Let us know what happens.
Again, perform the same network activity just before the waiting message comes up. Let us know what happens.
I know that there is a JRiver event raised at times that has "waiting" in the event name but have never paid much attention to it. Will try to dig deeper into it.