Noticed this today (macOS updated yesterday). Plexamp on my iPhone and iPad work perfectly; Plex desktop on macOS works perfectly (playing the 3 music libraries).
Upon starting (on macOS) the Plexamp window briefly shows some music albums, then the screen reverts to “WHOOPS. We had an issue getting data from [Mymediaserver], and we’re pretty bummed about it” plus a RETRY button (which doesn’t fix it).
Also, I note that in settings->source, I can only choose from 2 out of my 3 servers.
I tried reverting to v4.8.0 and that doesn’t work on macOS 13.5 either.