I’m experiencing this too intermittently controlling headless Rpi plexamp 4.3.0 from macos plexamp 4.3.0.
Here’s another data point: it just happened to me and while I was searching around to find where plexamp on macos stores its logs, the next song in my playlist started and that fixed the pause function. I was unable to get pause working on the last track, then the track changed and pause is working again.
Anyone know where mac plexamp logs to?
EDIT: That’s funny (and cool). Discourse is turning my typing plexamp logs 
into a link to a help post that shows where they are.
