I love the Plexamp app, but found that with the 3.1.0 version for MacOS. The skip buttons are hit or miss if they work.
The buttons will work if I am lucky for 3 skips, sometimes less. it seems to be related to how quickly the skip button is clicked.
Then the app locks, with everything in the window disappearing. Showing only a grey screen, requiring a force quit to exit the app.
I have let it go for over an hour and the app never comes back.
I hope that you can fix this in the next release.