Edit: Never mind. I got it working.
I tried using AirPlay but that didn’t work either so I toggled Force AirPlay 1 off and rebooted RoPieee
Now AirPlay and Plexamp both work. 
I can connect to RoPieee but when I try to play music Plexamp either opens the album page for a moment and then it closes or the app hangs on a buffering screen.
I read in the release notes for Plexamp Ver. 4.10.0 that there have been changes to headless Plexamp.
Could this be the cause?
I’m running RoPiee 1396 and it’s been a few months since I’ve used Plexamp. I tried entering a new claim code but it didn’t help.
Thanks.