I had once experienced the "album art only" issue, but can't recall how I got around it other than to power off then on my av8802.
Yeah, using the Marantz UI to "pull" from JRiver is not fun (ugly UI).
Try these methods:
1) In Media Center app, do you see your Marantz listed in the "Playing Now" section, on the left?
If so, click it to select it. Now you can:
1a) Use the "Play Doctor" on the right to find things to play on your Marantz, or
1b) Drag files to the Marantz item under Playing Now in the upper left. How? Well, try using the "Audio" or "Playlists" sections in that same left pane to find some music, then drag the items to the "Marantz" item in the upper left. It should start playing there.
Or...
2) Use the built-in Gizmo web app. On the computer running JRiver, just hit this in your browser:
http://localhost:52199/Gizmo/library.htmlIt may take you to a getting started screen, or it'll just say at the top "Home" and something like "Playing here (click to change)". Tap or click that to get to a list that shows your zones, including your Marantz. Choose the Marantz on that screen to target it. Now when you use the web UI it'll play songs to the Marantz. The web ui lets you brows playlists, search, etc. Just tap a track or "Play All" on a given screen to start playing.
Both options are far less ugly than the Marantz built in screen!
Good luck and let me know if you run into trouble.