Plexamp 3.0.0 is functional on Ubuntu 18.04 - I’m able to play music. I don’t see any images at all in the interface, though.
Edit: For anyone else running into this, I had to wipe out the files from the old version of Plexamp and try again:
rm -r ~/.cache/Plexamp
rm -r ~/.config/Plexamp
rm -r ~/.local/share/Plexamp
Now it’s working as expected for me.
It’s probably more likely that it was just a first-run glitch.